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Contents ~ s - t~Page
1 Introduction 83 2 Calculation of the Vertical Veloc1ty 86 3 Discussion of the Results 91
References 99
H FLOHN
ELEMENTS OF A SYNOPTIC CLlMATOLOGY OF THE
INDO-PAKISTAN SUBCONTINENT
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Abstract
From synoptic and aerologie data - together with many pictures from meteorological satellites - tbe seasonal varying largeshy
scale flowpatterns above the lndo-Pakistan subcontinent are outlined together with the rain frequency distribution Special consideration is given to the development of summer monsoon winds and rainfall patterns and the differences
between tbem and to tbe role of tbe Tibetan highlands in these processes
Zusammenfassung
Auf der Grundlage synoptischer und meteorologischer Daten
sowie einer groszligen Zahl Von Wolkenbildern meteorologischer Satelliten werden die jahreszeitlich wechselnden groszligshy
raumlumigen Stroumlmungsverteilungen uumlber dem indo-pakistanischen Subkontinent dargestellt und gleichzeitig damit die Vershyteilung der Niederschlagshaumlufigkeit Besonderes Gewicht
wird auf die Untersuchung der Entwicklung der Sommermonsunshy
Winde und -Niederschlagsverteilung die dabei auftretenden Differenzen und die Rolle des Tibetanischen Hochlandes in diesen Prozessen gelegt
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Elements of a Synoptic Climatology of the
lndo-Pakistan Subcontinent
The availability of aerological soundings for the lndo-Pakistan
Subcontinent - especially since the introduction of RAWIN Techniques and since the establishment of an aerological
network in the Chinese Peoples Republic including Tibet (1956) - has greatly improved our knowledge of the synoptic climatology of that area For a complete understanding we
have to include tbe whole area from the Persian Gulf and Pamir Mts to the western provinces of China from northern Tibet to the oceanic areas south of the equator
Our attempt to outline the large-scale synoptic features
responsible for tbe weather and climate of this vast area is partly based on extensive but hitherto unpublished investigations dealing with the summer season from my collaborators Dipl Met F ALFUTH Dr M WAGNER using aerological data of 1953 andor
1956 Mr HARBHAJAN ANAND M Sc (Tibet) and Dipl Met E RUPRECHT (Tharr Desert) Many studies by Indian meteorologistics
have also been published which are far too numerous to be quoted here the reader may refer to tbe Indian Journal of Meteorology and Geophysics and to three proceedings of symposia+)
Since 1949 the author has given several reviews on the Indian
Summer Monsoon (FLOHN 1950 1956 1960 1965 1964) and on the peculiar summer climate of Tibet (FLOHN 1958 1959 1968) in substance this report is a largely revised and extended
version of FLOHN (1965) An inspection of many tbousands of satellite pictures from tbis area (FLOHN 1968 1970) has much enlarged the limited personal experience of the author
+) 1) Monsoons of the World India Meteor Dept New Delhi
1960 2) Proceedings of the Symposium on Tropical Meteorology
Rotorua N Z 1963 3) Proceedlngs of the Symposium on Meteorological Results
of the Indian Ocean Expedition Bombay 1965
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A) During winter - especially from mid-December to February shy
upper westerlies prevail at the maximum wind level near 200 mb
above the whole area north of latitude ISoN and in all layers
above the Punjab and northernmost India They may even reach
the southern tip of the peninsula Below the westerlies the
well-known north-east monsoon - whieh mayaiso correct1y be
defined as the north-east trade - is quite shallow (1 - 15 km)
in the latitudinal belt between 220 and 300 N its vertical
depth increases towards the south above the peninsula Oue to
the Foumlhn effeet of the surrounding high mountains this
north-east monsoon eonsists of dry eontinental air diverging
and subsiding as in most trade wind areas
Within the upper westerlies the subtropical jet ean be obshy
served nearly every day between latitude 250 and 30o
N with
frequent troughs and ridges (Rossby waves) moving fair1y
rapidly in an easterly direetion from North Africa to southern
China (KRISHNAMURTI 1961 RAMAGE 1952) In the advanee
seetion of the troughs where vertical stretching prevails
together with upper divergenee and lower eonvergenee
(RAMASWAMY 1956) we observe not_ infrequently strong eonshy
veetive activity whieh is indieated at first by Ae cast or
Ae floe then by large towering eumulonimbus (with strong
shear in the forerunning anvil) Due to the extremely dry
air with eumulus bases frequently situated at 4 or 5 km
a large part of the falling rain evaporates before reaching
the ground but heavy gusts thunder and lightning and in
arid regions dust or sand storms are not rare events On
the warm southern side of the subtropieal jet preeipitation
eonneeted with these systems is in most eases only weak In
the north-west of the subeontinent espeeially in northern
Punjab and Baluchistan and in the Himalayas and their footshy
hills these Christmas rains (equivalent to the Mediterranean
winter rains) form adefinite rainy season varying in intensity
largely from year to year These rains are also responsible
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for the high snow cover of the Hindukush of the Karakorum
and of the north-western Himalayas (FLOHN 1969) These high
precipitations are by no means reflected in the measurements
of valley stations like Leh or Gilgit here the regular
occurence of daytime upslope breezes diverging from the
valley bottom prevents substantial precipitation (SCHWEINFURTH
1956) Only on rare oeeasions do these winter rains extend
into the Indian peninsula but then may reach as far as lSoN
The weak orographie eurvature of the Himalayas seems to be
responsible for the frequent occurrenee of troughs near 900 E
(Gulf of Benga1) where the average wind direction shifts
from 2S0-2900 in the west to about 260-2700 in the east The
ro1e of the Himalayas during this season can be interpreted
as a persistent cold source due to the eooling power of the
elevated snow-cover in the layer 3 - 7 km (or 700 - 400 mbs)+
Together with the warm subtropieal low-lands of northern India
this cold source forms a strong baroelinic zone probably
together with a weak direct meridional ce1l with rising warm
air and subsiding cool air In the northern Punjab - as weIl
+ According to satellite pictures (ESSA 3 and 6 January 1967
and FebruaryjMareh 1965) the eoverage of permanent snow
in the Himalayas east of 780 E is usually only 30-50 percent
in contrast to Karakorum and Hindukush mountains with more
than 70 percent Large areas of southern Tibet - especially
between 7SoE and 920 E and south of 300 N - are snow-free on
about SO percent of all days The other 20 of the days
travelling disturbances produce clouds and snow which
disappears rapidly due to strong daytime radiation and
evaporation Even centra1 Tibet appears to be snow-free in
about 50 percent of all cases (FLOHN 1968) In contrast to
the marginal mountains the highlands of southern and cen~ral
Tibet ean act as an elevated heat source even during winter
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as in adjacent Afghanistan and Baluchistan _ the winter rains
increase in intensity (and sometimes also in frequency six to
ten days a month) during March and April but decrease during May
The above-mentioned direct meridional cell is not reflected
in the precipitation distribution In fact th~ average preshy
cipitation in the mountains is much greater due to orography
than in the plains Recent investigations on the water balance of the Hindukush and adjacent mountains have shown that the
winter-spring snowfall in the mountains is much greater than
hitherto estimated (FLOHN 1969) This confirrns other studies
on the mass balance of the enormous glaciers of the Karakorurn the Pamirs etc
B) During spring the distribution of the upper westerlies
is nearly the same as during winter including the existence
of the quasi-permanent Bengal trough But the increasing solar
radiation leads to the development of shallow but persistent
heat lews above the southern part of the peninsula which
gradually move to the north and eventually reach (in early
June) the Punjab On their southern side shallow and weak
westerlies - which during the last century were falsely
called sea-breezes - occur with little or no effect on
weather Over the Bay of Bengal and across its northern coast
weak southerly (SW-SE) winds with moist maritime air gradushy
ally replace the dry north-east monsoon Now the divergence
aloft in front of a travelling upper trough of the subtropical jet frequently coincides above north-east India (Bengal
Assam East Pakistan and perhaps also Upper Burma) with a
shallow layer of unstable moist air a situation which
typically produces severe storms Therefore from mid-March
to May the squalls of the Bengal Nor-westers increase
greatly in frequency intensity and rainfall With this
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synoptic mechanism the rainy season of north-eastern India
starts during spring much earlier than the marked reversal
in the upper winds At many places the nurnber of rainy days
(with 25 rnrn and more) increases to 12-19 per month due to
the orographical trapping of the low-level southerly flow in
the Assam area which is surrounded on three sides by high
mountains During April and May (FtoHN 1968) 10 typical synshy
optic stations in the Assam plains receive as much as 24 rnrn
per rainy day the whole area receives as much as 88 rnrn per
day regardless of the nurnber of stations with rain and in
only 7 percent of all cases is the area rainless (The same
nurnbers for northern Germany during summer are 50 rnrn 30 rnrn
and 10 percent respectively)
Over the Indian Ocean some tropical cyclones are observed
most of which can be followed from the South China Sea or
the Gulf of Thailand as they progress to the eoast Occasioshy
nally they develop into full-size tropical hurricanes with
warm eores and with anticyclonic flows above 200 rnb They are
then steered toward the north-west or even north in the
advance section of a westerly trough and are accompanied by
torrential rains and floods Despite their rareness they
may even re-oceur after a week or so and are sometimes
accompanied by disastrous flood effects in the densely popushy
lated low-level area in the delta of the Ganges and Brahrnashy
putra
Cl During May and early June the seasollal warming of the air
above NE-India - produeed by the release of latent heat from
precipitation processes - and above southern Tibet - here
caused by the elevation of the heated surface near 4500 m
(equivalent to about 580 rnb) - weakens and finally reverses
the meridional gradient of temperature between subtropical
latitudes and the equator (FLOHN 1964) According to
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aero1ogieal data this proeess starts from the uppermost troposphere (150-200 mb) and proeeeds slow1y downwards The
reversal of this meridional temperature gradient in the 1ayer 800-500 mb oecurs almost simu1taneous1y with the onset of monaoon rains along the western eoast of the peninsula This results - almost simultaneously with the northward displaeement of the low-level heat low - in a complete
reversal of the upper winds ( between about 450 and 100 mb)
from west to east a rapid inerease of the vertical depth of the equatorial westerlies now known as the south-west monsoon to 6-7 km and their intrusion into the low-pressure area extending from the Punjab to Assam and further to the eastshy
south-east This syndrome of nearly simu1taneous events is observed under normal conditions during the first ten days
of June in some years it oeeurs already during late May in others it may be postponed unti1 late June It is wel1-known
in classical textbooks as the burst of the monsoon But after recognizing the lack of coincidence between rainfall
distribution thunder-storm frequency and three-dimensiona1 wind patterns we see that it is actually quite a diffieult job to define exaet1y one (and only one) date for this event
at a given point The literal translation of the Arabic term maus im is quite generally season therefore this term has originally no specifie meaning like monsoon winds or monsoon rains The lack of coincidenee between monsoon winds
and monsoon rains is responsible for frequent misinterpreshy
tations and confusion the onset of the rainymiddot season is eontrolled in different areas by quite different synoptic
features
This reversal of the three-dimensional wind field 1a aceompanied or better triggered by the occurrence of anticyclonic cells in the divergence zone between the high-tropospheric eastershylies and the westerlies near latitude 30oN The average
position of the subtropical jet shifts to about 40oN with a
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weak anticyelonic curvature on the northern fringe of the Tibetan highlands This rapid displacement coincides with a re-arrangement of the quasi-permanent troughs (Figure 1)
Instead of the weak cyclonie eurvature of the subtropical jet near 900 E (Bengal trough) we now observe two rather
marked troughs just upstream and downstream of the Tibetan block the Pamir trough (near 650 E) and the west Chinese (or
Szechwan) trough (at lOO-10SoB)
We might remark at this point without further explanations that nearly simultaneously the quasi-permanent secondary trough over eastern Europe (centred around 300 E) disappears
together with a rapid decrease of the number of b10cking anticyclones above north-western Europe a consequence of
Rossbys well-known stationary wave formula This causes a marked singularity of weather in large parts of Europe
unfortunately misnamed the European summer monaoon Furthershy
more at this time the occurrence of a quasi-stationary antishycyclonic cell above the cool Okhotsk Sea together with the
sharpening of the Szechwan trough is responsible for the occurrence of Bai-U - rains above central and northern
China and Japan (ASAKURA 1968) which are mainly produced by weak cyclones running along the marked baroclinic frontal zone (Pacific Polar Front) Wh1le these two events are
certainly related in a dynamical sense to the events between Pamir and Szechwan this 1s probably not the case with respect
to a third event the onset of Arizona summer rains which apparently have many parallels to the simultaneous developshyments in the Punjab These large scale teleconnections deserve a comparative investigation
The usual distinction between an_Arabian branch and a Bengal branch of the south-west monsoonis somewhat misleading In
both areas westerly winds of themiddot lower troposphere - mostly
250-2900 - converge along a slowiy displacing convergenoe
_____ _
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4~ m1c-1
~_--15
10 0-000 SIIt1f11lf bullbullbullbull Co~1 S a~orl I bull
bull I Sbtntpual _1 bullbull is lJlff tnhl9h AXIS
Fig 1 Position of the Fig 3 Meridional crossshysubtropical and of the section of zonal winds semi-permanent troughs along Long 780 E (arrows = during summer and during upper air stations) Julyshythe cool seasons August 1961-62 (October-May) bull
Fig 2 Position of the monsoon pressure trough (= northern
branch of the ITC) and streamlines of the resultant
wind at 09 kms during summer
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line with a deep current from east-south-east (Figure 2)
which extends right up to the easterlies of the upper troposhy
sphere At the southern edge of this convergence - which forms
part of the main ITCZ (northern branch) and the equatorial
pressure trough - isolated cyclonic disturbances without
distinct fronts and air-masses frequently travel from eastshy
south-east to west-north-west producing much rain in their
southern and western sections These monsoon depressions
are responsible for the bulk of the summer rains in the
interior of the lndo-Pakistan subcontinent Their forecast
depends mainly on sufficient knowledge of the rather comshy
plicated three-dimensional wind field in that area up to at
least 200 mb Not infrequently these disturbances can be
followed during their earlier track from the east Apart
from these low-level cyclones frequent strong convective
rains are observed in two regions in Bengal and Assam where
the low-tropospheric south-west flow permanently converges
with the easterlies (including orographically foreed lifting
at the Khasia Hills) and at the steep escarpments of the
western eoasts of both South-Asian peninsulae But even along
these eoastal escarpments large variations from day to day
oeeur probably related to variations of the divergenee of
low-level winds The strong contribution of eonvective
activity- interrupted by short rainless subsidence periods shy
to the summer rain in Nepal has been demonstrated by DITTMANN
(1970)
On the average the boundary between the extra-tropical
westerlies and the easterlies above the south-west monsoon
(Figure 3) is situated between 400-100 mb (7-16 km) near
300 N ie just above a large part of the Himalaya mountains
At the same time the subtropical jet is situated at latitude
40-420 N ie above the Tarim Basin as revealed by Chinese
aerological data with a rernarkable constaney The boundary
between these currents is marked by well-defined anticyclonic
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cells frequently centered in the south-eastern portion of
the Tibetan highlands Sometimes the travelling upper troughs of the westerlies may penetrate near 200 mb to about latishy
tude 26-27oN but hardly ever farther south If~ in such a case a low-level monsoon depression approaches from southshy
east it can be intensified and steered into a northerly
direction and eventually merge with the trough Then torrential
rains floods and landslides are observed in the Himalayas
while the divergence of the west-south-west flow above the
peninsula produces widespread subsidence and suppression of
convective activity This is at least one of the mechanisms
responsible for the well-known monsoon breaks (RAHMATULLAH
1952 RIEHL 1954) In fact if we disregard the relatively
small areas of orographically forced convective rains and of
permanent convergence in the north-eastern portion of the
subcontinent we may safely say that ove~ the largest part
of the Indo-Pakistan area the summer rains are produced by
well-differentiated synoptic events and cannot be conceived
as a more or less steady climatic feature Even the quasishy
permanent anticyclonic cell above south-eastern Tibet (FLOHN 1958 1968) can be swept away by a travelling westerly trough
with a cold air outbreak across Tibet it recovers however
after two to three days sometimes being reinforced by warm
air advection Thererore maps of rainfall frequency (SCHWEINshyFURTH et al 1970) are a necessary implementation of the
well-known maps of precipitation amount
The track of these monsoon depressions is controlled by the
upper flow or bette~ by integration of the wind field
horizontally and vertically with respect to pressure and
taking into account the well-known meridional variation of
the Coriolis parameter szlig = dfdy Under normal conditions
they are therefore steered from east-south-east to west-northshywest only gradually diminishing in intensity and rainfall
productivity mostly as fairly symmetrie vortices near the
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o0 I I Irltt I 60 0 110deg
Flow Patterns 800-900mbs-+
100 - 300 IIE-
Cold Warm Co Id Warm100 I mbs INDIRECT
500 CELL GEClCELL
Conv900 Div Div Conv I
A A B 8
Fig 4 Large-scale features of the Indian Summer Monsoon
Above flow patterns in lower and upper troposphere Below cross-circulation along B - B (entrance)
and A - A (exit region of the Tropical Easterly Jet)
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S50 roh level Apparently lifting and consequently rainfall
reach a maximum in their southern section there is hardly
sufficient evidence of definite air-mass differences in their
area Generally speaking air-mass differences during the
monsoon season above the subcontinent are much smaller than
in middle latitudes and hardly detectable in the lower
troposphere However in the upper troposphere we observe a remarkable baroclinie structure where the equatorial air
above SON is about 7-10oC cooler than the subtropical air
above the heat eentre at 28-30oN This reversed temperature
gradient is eorrelated with an extremely persistent tropiealo
easterly jet (FLOHN 1964) centred near latitude IS N one or two km below the tropieal tropopause This easterly jet is
of remarkable persistence from day to day with only oecasional fluetuations of the wind velocity (pulses) whieh
have a duration of three to six days and varying between
about 40 and more than 100 knots in the centre According to
recent studies by RAMAN (1964) these pulses are eorrelated
with fluctuations of the rainfall above the subcontinent and thus with the varying release of latent heat which
intensifies the anticyelonie eells above the Himalayas
On the average the speed of the Tropical Easterly Jet inshy
creases (together with a northerly component of the winds)
east of 850 E and decreases (together with a weak southerly
eomponent) towards the west This causes (FLOHN 1964) a eross-isobaric meridional circulation reverting its sign
near 8SoE (Figure 4) Above SE Asia ascending motion with
strong cloudiness and rainfall prevails along the northern edge of the system (Lat 20-32oN) while subsiding motions
are frequently found along th~ southern edge near the equator Above SW-Asia and Africathe reverse is true (FLOHN
1964) large-scale subsidence tendency along the northern flane (including the area of the Intertropical Convergence
Zone at the surface) in contrast to lifting motions along
~
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the southern edge (the belt of highest rainfall along Lat
7-14oN)
Nevertheless climatic features near surfaee are by no means
unimportant in the large-scale meehanism of the summer-monsoon
rains in southern Asia We list only a few of them
a) The reversal of the temperature pressure and wind field
of the middle and upper troposphere is preceded by the gradual
warming of northern India the Tibetan highlands and the
adjaeent highlands Bere we have to distinguish two main
sources of internal and potential energy
1 The flux of sensible heat into the air from the elevated
heat souree of the arid (western and central) seetion of
the Tibetan highlands Here all terms of the heat balance
can be assumed to be nearly equal to those in the adjacent
arid lowlands of centra1 Asia Suffieient evidence for
this assumption has been presented by AIZENSHTAT (1966)
from many heat balance investigations in the arid high
mountains of the USSR at altitudes of 3100-4000 m The
average input of sensible heat into the atmosphere amounts
to 180-250 Lyday (1 Ly = 1 calcm2)
2 The release of latent heat from the enormous quantities
of rain falling on the superhumid mountain areas of Bengal
Assam and Upper Burma This has an estimated area-averaged value for the cloud layer of 900-1200 Lyd ie more
than the global radiation at the surfaee can yield Its
effect has been suggested generally (FLOHN 1953) as an
addition to the anticyclonic curvature of the upper westershy
lies along high mountain ridges Oue to the rapid decrease
of moisture with height it will increase the thiekness of
the lower troposphere however the aseending motion in the
eb towers carries much heat upward to the 150-300 mb layer
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Most of this latent heat is used for heating up the upper
troposphere to a moist-adiabatic lapse-rate Since in SEshy
Tibet and Assam the average vertical lapse-rate is slightly
super-moist-adiabatie any ascending parcel of air in the
eondensation layer between 775 and 205 mb is about 16oC
warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in
the arid areas
The effect of the direct warming of air above southernmost
and western Tibet and north-eastern India has been qualitashy
tively stressed by the author (1950 1960 1968) Strong
evidenee has been contributed in a model eomputation by
MURAKAMI (1958) who started from the average 300 mb-flow
during May introduced a heat source above the whole
Tibetan plateau and obtained after a model time of 72
hours an antieyelonic eell in southeastern Tibet In a
more recent model of a two-dimensional meridional cross-
section along BOoE the role of the mountains and of the
hydrologie cycle has been investigated numerically by the
same author (1969) Using correlation analysis ASAKURA
(196B) recently demonstrated the strong persistence and
the large-seale influence of the Tibetan anticyelone at
the 500 mb-level
Careful analysis of all available aerological data shy
taking Into account the diurnal variation of upper air
temperatures as well as thermaI winds (FLOHN 196B) shy
revealed beyond any doubt the existenee of an elongated
heat zone in the upper troposphere (300-500 mb) above the
Himalayas and southern Tibet centered at 300 N and 850S
(Figure 5)
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- 22 shy
b) In the arid north-western part of the subcontinent (Sind
Rajasthan) the aridity is maintained even during the monsoon
season when moist air with an almost constant dewpoint near 0
25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days
when monsoon depressions travelling from the east or south
penetrate this area On all other days - ie during more
than 90 per cent of the season - the permanent and steady
monsoon flow is divergent turning from south-west above
Jodhpur and Gwalior to south-south-east above Jacobabad and
therefore subject to large-seale subsidence (FLOHN et al
196B) which reduces the vertical extension of the moist
layer to about 15-2 km and its eloud cover to shallow
stratocumuli (sc) This divergence can be understood if we
take into aceount the average position of the pressure trough
with its anticyelonic curvature from eastern Arabia to the
Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1
Similarly the SW-monsoon above the Arabian Sea is - at least
west of Long 65-68o
E - essentially rainless with a shallow
layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by
directional diffluence but by the increasing velocity along
the stream lines About 500-BOO kms off the Indian west coast
the wind speed decreases accompanied by a remarkable increase
of shower activity These facts strongly support our present
view of the dynamic effects of the wind vergences in contrast
to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)
cl The remarkable Foumlhn effect on the eastern side of the
West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a
frequency of 29-30 rain-days a month from June to September)
we observe only 20-40 km east of the crest not more than
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600-800 mm mostly produced by a few travelling synoptic
disturbances In some of the deepest valleys of the Himalaya
Mountains - even in the rainiest areas north of Assam - the
valley bottom is almost arid (as demonstrated by the vegeshy
tation) (SCHWEINFURTH 1956) since the local circulations
always control the pattern of wind components clouds and
rain (FLOHN 1970 DITTMANN 1970) This is also true in the
deep meridional gorges of Upper Burma - in spite of the high
cloudiness and moisture content of the air during late spring
and summer below the quasi-permanent anticyclonic cell in the
upper troposphere The slight anticyclonic curvature of the
lower westerlies crossing the Ghats (Figure 2) might also be
related to the permanent release of latent heat
d) The distribution of land- and sea-breezes controls a large
part of the convective activity The frequency and extend of
these diurnal circulations along all the coasts of the subshy
continent deserves much more attention together with their
effect on the weather During the summer monsoon these
diurnal circulations are to a large extend weakened or even
suppressed During the remaining seasons the quasi-geostrophic
flow near the surface is much weaker the diurnal radiation
cycle is more effective due to low cloudiness and the diurnal
circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)
middote) Careful analysis of the diurnal wind circulation at the
Himalayas led to the unexpected conclusion that the usual
reversal from the ascending day-time circulation to the
descending nighttime circulation is to a large extend
suppressed by the permanence of the above-mentioned heat
center in the upper troposphere during night (FLOHN 196B
1970) Thus in many valleys in the Himalayas (including the
meridional gorges) the weak nighttime mountain winds are
either superimposed with upstream winds or are replaced by calms
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D The retreat of the monsoon during September and October
(REITER amp HEUBERGER 1960) seems not to be merely a mirror
of the events during the onset in early June The large-scale
reversamiddotl of the wind field is somewhat more gradual and
relatively slow but low-Ievel cyclones still cross the
peninsula and may be steered in a northward direction causing
veritable cloudbursts and severe floods Due to the gradual
~~ifting of their tracks they often have the opportunity to
ltevelop over ocean areas into full-size warm-core hurricanes
notably over the Gulf of Bengal From October to December
tropical hurricanes and other cyclones of minor intensity
are fairly frequent here they can almost certainly be tracked
along their path from the east ie from the Gulf of Thailand
They are responsible for the late fall rains on the east coast
(and over the southcentral part) of the peninsula as weIl as
for the rainy season on all eastfacing coasts of south-east
Asia In contrast to spring they are in this season relatively
rare (but not absent) over the Arabian Sea Since the northshy
east flow itself is shallow and mostly stable at least north
of about latitude 12oN its effect on rainfall is small The
fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations
from year to year Since the cyclonic disturbances usually
travel towards west the shearing intensity reaches its maxishy
mum at their western side Together with the increase of
surface stress at the east-facing coasts this leads to a
marked rainfall maximum along these coasts
E In order to avoid further misunderstanding it should be
stressed that the monsoonal reversal of the surface winds is
by no means identical with begin and end of the rainy season
In fact the rainy season starts in the northeastern area of
the subcontinent about two months earlier than the reversal
of (upper) winds while in the heart of the peninsula the
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low-level wind shift precedes the onset of the monsoon rains
by about the same time
The early onset of rains in NE-India contributes - together
with the seasonal warming of the elevated Tibetan highlands shy
to the development of the high-tropospheric summer anticyclones
above the Himalayas This event produces the reversal of the
winds of the mlddle and upper troposphere which now triggers
the onset of the monsoon rains above the largest portion of
the subcontinent Such an interaction between thermodynamic
and dynamic processes is by no means uncommon in the largeshy
scale atmospheric circulation In this area it is not merely
a seasonal shift but an orographically fixed reversal of
temperature and pressure gradients which leads to such dramatic
(and far-reaching) consequences Before the existence of the
Tibetan highlands - ie before the late-Tertiary (and recent)
uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa
or Australia This is also evident from the recent model
computation of MURAKAMI (1969)
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
- 6 shy
Abstract
From synoptic and aerologie data - together with many pictures from meteorological satellites - tbe seasonal varying largeshy
scale flowpatterns above the lndo-Pakistan subcontinent are outlined together with the rain frequency distribution Special consideration is given to the development of summer monsoon winds and rainfall patterns and the differences
between tbem and to tbe role of tbe Tibetan highlands in these processes
Zusammenfassung
Auf der Grundlage synoptischer und meteorologischer Daten
sowie einer groszligen Zahl Von Wolkenbildern meteorologischer Satelliten werden die jahreszeitlich wechselnden groszligshy
raumlumigen Stroumlmungsverteilungen uumlber dem indo-pakistanischen Subkontinent dargestellt und gleichzeitig damit die Vershyteilung der Niederschlagshaumlufigkeit Besonderes Gewicht
wird auf die Untersuchung der Entwicklung der Sommermonsunshy
Winde und -Niederschlagsverteilung die dabei auftretenden Differenzen und die Rolle des Tibetanischen Hochlandes in diesen Prozessen gelegt
- 7 shy
Elements of a Synoptic Climatology of the
lndo-Pakistan Subcontinent
The availability of aerological soundings for the lndo-Pakistan
Subcontinent - especially since the introduction of RAWIN Techniques and since the establishment of an aerological
network in the Chinese Peoples Republic including Tibet (1956) - has greatly improved our knowledge of the synoptic climatology of that area For a complete understanding we
have to include tbe whole area from the Persian Gulf and Pamir Mts to the western provinces of China from northern Tibet to the oceanic areas south of the equator
Our attempt to outline the large-scale synoptic features
responsible for tbe weather and climate of this vast area is partly based on extensive but hitherto unpublished investigations dealing with the summer season from my collaborators Dipl Met F ALFUTH Dr M WAGNER using aerological data of 1953 andor
1956 Mr HARBHAJAN ANAND M Sc (Tibet) and Dipl Met E RUPRECHT (Tharr Desert) Many studies by Indian meteorologistics
have also been published which are far too numerous to be quoted here the reader may refer to tbe Indian Journal of Meteorology and Geophysics and to three proceedings of symposia+)
Since 1949 the author has given several reviews on the Indian
Summer Monsoon (FLOHN 1950 1956 1960 1965 1964) and on the peculiar summer climate of Tibet (FLOHN 1958 1959 1968) in substance this report is a largely revised and extended
version of FLOHN (1965) An inspection of many tbousands of satellite pictures from tbis area (FLOHN 1968 1970) has much enlarged the limited personal experience of the author
+) 1) Monsoons of the World India Meteor Dept New Delhi
1960 2) Proceedings of the Symposium on Tropical Meteorology
Rotorua N Z 1963 3) Proceedlngs of the Symposium on Meteorological Results
of the Indian Ocean Expedition Bombay 1965
- 8 shy
A) During winter - especially from mid-December to February shy
upper westerlies prevail at the maximum wind level near 200 mb
above the whole area north of latitude ISoN and in all layers
above the Punjab and northernmost India They may even reach
the southern tip of the peninsula Below the westerlies the
well-known north-east monsoon - whieh mayaiso correct1y be
defined as the north-east trade - is quite shallow (1 - 15 km)
in the latitudinal belt between 220 and 300 N its vertical
depth increases towards the south above the peninsula Oue to
the Foumlhn effeet of the surrounding high mountains this
north-east monsoon eonsists of dry eontinental air diverging
and subsiding as in most trade wind areas
Within the upper westerlies the subtropical jet ean be obshy
served nearly every day between latitude 250 and 30o
N with
frequent troughs and ridges (Rossby waves) moving fair1y
rapidly in an easterly direetion from North Africa to southern
China (KRISHNAMURTI 1961 RAMAGE 1952) In the advanee
seetion of the troughs where vertical stretching prevails
together with upper divergenee and lower eonvergenee
(RAMASWAMY 1956) we observe not_ infrequently strong eonshy
veetive activity whieh is indieated at first by Ae cast or
Ae floe then by large towering eumulonimbus (with strong
shear in the forerunning anvil) Due to the extremely dry
air with eumulus bases frequently situated at 4 or 5 km
a large part of the falling rain evaporates before reaching
the ground but heavy gusts thunder and lightning and in
arid regions dust or sand storms are not rare events On
the warm southern side of the subtropieal jet preeipitation
eonneeted with these systems is in most eases only weak In
the north-west of the subeontinent espeeially in northern
Punjab and Baluchistan and in the Himalayas and their footshy
hills these Christmas rains (equivalent to the Mediterranean
winter rains) form adefinite rainy season varying in intensity
largely from year to year These rains are also responsible
- 9 shy
for the high snow cover of the Hindukush of the Karakorum
and of the north-western Himalayas (FLOHN 1969) These high
precipitations are by no means reflected in the measurements
of valley stations like Leh or Gilgit here the regular
occurence of daytime upslope breezes diverging from the
valley bottom prevents substantial precipitation (SCHWEINFURTH
1956) Only on rare oeeasions do these winter rains extend
into the Indian peninsula but then may reach as far as lSoN
The weak orographie eurvature of the Himalayas seems to be
responsible for the frequent occurrenee of troughs near 900 E
(Gulf of Benga1) where the average wind direction shifts
from 2S0-2900 in the west to about 260-2700 in the east The
ro1e of the Himalayas during this season can be interpreted
as a persistent cold source due to the eooling power of the
elevated snow-cover in the layer 3 - 7 km (or 700 - 400 mbs)+
Together with the warm subtropieal low-lands of northern India
this cold source forms a strong baroelinic zone probably
together with a weak direct meridional ce1l with rising warm
air and subsiding cool air In the northern Punjab - as weIl
+ According to satellite pictures (ESSA 3 and 6 January 1967
and FebruaryjMareh 1965) the eoverage of permanent snow
in the Himalayas east of 780 E is usually only 30-50 percent
in contrast to Karakorum and Hindukush mountains with more
than 70 percent Large areas of southern Tibet - especially
between 7SoE and 920 E and south of 300 N - are snow-free on
about SO percent of all days The other 20 of the days
travelling disturbances produce clouds and snow which
disappears rapidly due to strong daytime radiation and
evaporation Even centra1 Tibet appears to be snow-free in
about 50 percent of all cases (FLOHN 1968) In contrast to
the marginal mountains the highlands of southern and cen~ral
Tibet ean act as an elevated heat source even during winter
- 10 shy
as in adjacent Afghanistan and Baluchistan _ the winter rains
increase in intensity (and sometimes also in frequency six to
ten days a month) during March and April but decrease during May
The above-mentioned direct meridional cell is not reflected
in the precipitation distribution In fact th~ average preshy
cipitation in the mountains is much greater due to orography
than in the plains Recent investigations on the water balance of the Hindukush and adjacent mountains have shown that the
winter-spring snowfall in the mountains is much greater than
hitherto estimated (FLOHN 1969) This confirrns other studies
on the mass balance of the enormous glaciers of the Karakorurn the Pamirs etc
B) During spring the distribution of the upper westerlies
is nearly the same as during winter including the existence
of the quasi-permanent Bengal trough But the increasing solar
radiation leads to the development of shallow but persistent
heat lews above the southern part of the peninsula which
gradually move to the north and eventually reach (in early
June) the Punjab On their southern side shallow and weak
westerlies - which during the last century were falsely
called sea-breezes - occur with little or no effect on
weather Over the Bay of Bengal and across its northern coast
weak southerly (SW-SE) winds with moist maritime air gradushy
ally replace the dry north-east monsoon Now the divergence
aloft in front of a travelling upper trough of the subtropical jet frequently coincides above north-east India (Bengal
Assam East Pakistan and perhaps also Upper Burma) with a
shallow layer of unstable moist air a situation which
typically produces severe storms Therefore from mid-March
to May the squalls of the Bengal Nor-westers increase
greatly in frequency intensity and rainfall With this
- 11 shy
synoptic mechanism the rainy season of north-eastern India
starts during spring much earlier than the marked reversal
in the upper winds At many places the nurnber of rainy days
(with 25 rnrn and more) increases to 12-19 per month due to
the orographical trapping of the low-level southerly flow in
the Assam area which is surrounded on three sides by high
mountains During April and May (FtoHN 1968) 10 typical synshy
optic stations in the Assam plains receive as much as 24 rnrn
per rainy day the whole area receives as much as 88 rnrn per
day regardless of the nurnber of stations with rain and in
only 7 percent of all cases is the area rainless (The same
nurnbers for northern Germany during summer are 50 rnrn 30 rnrn
and 10 percent respectively)
Over the Indian Ocean some tropical cyclones are observed
most of which can be followed from the South China Sea or
the Gulf of Thailand as they progress to the eoast Occasioshy
nally they develop into full-size tropical hurricanes with
warm eores and with anticyclonic flows above 200 rnb They are
then steered toward the north-west or even north in the
advance section of a westerly trough and are accompanied by
torrential rains and floods Despite their rareness they
may even re-oceur after a week or so and are sometimes
accompanied by disastrous flood effects in the densely popushy
lated low-level area in the delta of the Ganges and Brahrnashy
putra
Cl During May and early June the seasollal warming of the air
above NE-India - produeed by the release of latent heat from
precipitation processes - and above southern Tibet - here
caused by the elevation of the heated surface near 4500 m
(equivalent to about 580 rnb) - weakens and finally reverses
the meridional gradient of temperature between subtropical
latitudes and the equator (FLOHN 1964) According to
- 12 shy
aero1ogieal data this proeess starts from the uppermost troposphere (150-200 mb) and proeeeds slow1y downwards The
reversal of this meridional temperature gradient in the 1ayer 800-500 mb oecurs almost simu1taneous1y with the onset of monaoon rains along the western eoast of the peninsula This results - almost simultaneously with the northward displaeement of the low-level heat low - in a complete
reversal of the upper winds ( between about 450 and 100 mb)
from west to east a rapid inerease of the vertical depth of the equatorial westerlies now known as the south-west monsoon to 6-7 km and their intrusion into the low-pressure area extending from the Punjab to Assam and further to the eastshy
south-east This syndrome of nearly simu1taneous events is observed under normal conditions during the first ten days
of June in some years it oeeurs already during late May in others it may be postponed unti1 late June It is wel1-known
in classical textbooks as the burst of the monsoon But after recognizing the lack of coincidence between rainfall
distribution thunder-storm frequency and three-dimensiona1 wind patterns we see that it is actually quite a diffieult job to define exaet1y one (and only one) date for this event
at a given point The literal translation of the Arabic term maus im is quite generally season therefore this term has originally no specifie meaning like monsoon winds or monsoon rains The lack of coincidenee between monsoon winds
and monsoon rains is responsible for frequent misinterpreshy
tations and confusion the onset of the rainymiddot season is eontrolled in different areas by quite different synoptic
features
This reversal of the three-dimensional wind field 1a aceompanied or better triggered by the occurrence of anticyclonic cells in the divergence zone between the high-tropospheric eastershylies and the westerlies near latitude 30oN The average
position of the subtropical jet shifts to about 40oN with a
- 13 shy
weak anticyelonic curvature on the northern fringe of the Tibetan highlands This rapid displacement coincides with a re-arrangement of the quasi-permanent troughs (Figure 1)
Instead of the weak cyclonie eurvature of the subtropical jet near 900 E (Bengal trough) we now observe two rather
marked troughs just upstream and downstream of the Tibetan block the Pamir trough (near 650 E) and the west Chinese (or
Szechwan) trough (at lOO-10SoB)
We might remark at this point without further explanations that nearly simultaneously the quasi-permanent secondary trough over eastern Europe (centred around 300 E) disappears
together with a rapid decrease of the number of b10cking anticyclones above north-western Europe a consequence of
Rossbys well-known stationary wave formula This causes a marked singularity of weather in large parts of Europe
unfortunately misnamed the European summer monaoon Furthershy
more at this time the occurrence of a quasi-stationary antishycyclonic cell above the cool Okhotsk Sea together with the
sharpening of the Szechwan trough is responsible for the occurrence of Bai-U - rains above central and northern
China and Japan (ASAKURA 1968) which are mainly produced by weak cyclones running along the marked baroclinic frontal zone (Pacific Polar Front) Wh1le these two events are
certainly related in a dynamical sense to the events between Pamir and Szechwan this 1s probably not the case with respect
to a third event the onset of Arizona summer rains which apparently have many parallels to the simultaneous developshyments in the Punjab These large scale teleconnections deserve a comparative investigation
The usual distinction between an_Arabian branch and a Bengal branch of the south-west monsoonis somewhat misleading In
both areas westerly winds of themiddot lower troposphere - mostly
250-2900 - converge along a slowiy displacing convergenoe
_____ _
- 14 shy
4~ m1c-1
~_--15
10 0-000 SIIt1f11lf bullbullbullbull Co~1 S a~orl I bull
bull I Sbtntpual _1 bullbull is lJlff tnhl9h AXIS
Fig 1 Position of the Fig 3 Meridional crossshysubtropical and of the section of zonal winds semi-permanent troughs along Long 780 E (arrows = during summer and during upper air stations) Julyshythe cool seasons August 1961-62 (October-May) bull
Fig 2 Position of the monsoon pressure trough (= northern
branch of the ITC) and streamlines of the resultant
wind at 09 kms during summer
- l c shy
line with a deep current from east-south-east (Figure 2)
which extends right up to the easterlies of the upper troposhy
sphere At the southern edge of this convergence - which forms
part of the main ITCZ (northern branch) and the equatorial
pressure trough - isolated cyclonic disturbances without
distinct fronts and air-masses frequently travel from eastshy
south-east to west-north-west producing much rain in their
southern and western sections These monsoon depressions
are responsible for the bulk of the summer rains in the
interior of the lndo-Pakistan subcontinent Their forecast
depends mainly on sufficient knowledge of the rather comshy
plicated three-dimensional wind field in that area up to at
least 200 mb Not infrequently these disturbances can be
followed during their earlier track from the east Apart
from these low-level cyclones frequent strong convective
rains are observed in two regions in Bengal and Assam where
the low-tropospheric south-west flow permanently converges
with the easterlies (including orographically foreed lifting
at the Khasia Hills) and at the steep escarpments of the
western eoasts of both South-Asian peninsulae But even along
these eoastal escarpments large variations from day to day
oeeur probably related to variations of the divergenee of
low-level winds The strong contribution of eonvective
activity- interrupted by short rainless subsidence periods shy
to the summer rain in Nepal has been demonstrated by DITTMANN
(1970)
On the average the boundary between the extra-tropical
westerlies and the easterlies above the south-west monsoon
(Figure 3) is situated between 400-100 mb (7-16 km) near
300 N ie just above a large part of the Himalaya mountains
At the same time the subtropical jet is situated at latitude
40-420 N ie above the Tarim Basin as revealed by Chinese
aerological data with a rernarkable constaney The boundary
between these currents is marked by well-defined anticyclonic
- 16 shy
cells frequently centered in the south-eastern portion of
the Tibetan highlands Sometimes the travelling upper troughs of the westerlies may penetrate near 200 mb to about latishy
tude 26-27oN but hardly ever farther south If~ in such a case a low-level monsoon depression approaches from southshy
east it can be intensified and steered into a northerly
direction and eventually merge with the trough Then torrential
rains floods and landslides are observed in the Himalayas
while the divergence of the west-south-west flow above the
peninsula produces widespread subsidence and suppression of
convective activity This is at least one of the mechanisms
responsible for the well-known monsoon breaks (RAHMATULLAH
1952 RIEHL 1954) In fact if we disregard the relatively
small areas of orographically forced convective rains and of
permanent convergence in the north-eastern portion of the
subcontinent we may safely say that ove~ the largest part
of the Indo-Pakistan area the summer rains are produced by
well-differentiated synoptic events and cannot be conceived
as a more or less steady climatic feature Even the quasishy
permanent anticyclonic cell above south-eastern Tibet (FLOHN 1958 1968) can be swept away by a travelling westerly trough
with a cold air outbreak across Tibet it recovers however
after two to three days sometimes being reinforced by warm
air advection Thererore maps of rainfall frequency (SCHWEINshyFURTH et al 1970) are a necessary implementation of the
well-known maps of precipitation amount
The track of these monsoon depressions is controlled by the
upper flow or bette~ by integration of the wind field
horizontally and vertically with respect to pressure and
taking into account the well-known meridional variation of
the Coriolis parameter szlig = dfdy Under normal conditions
they are therefore steered from east-south-east to west-northshywest only gradually diminishing in intensity and rainfall
productivity mostly as fairly symmetrie vortices near the
- 17 shy
o0 I I Irltt I 60 0 110deg
Flow Patterns 800-900mbs-+
100 - 300 IIE-
Cold Warm Co Id Warm100 I mbs INDIRECT
500 CELL GEClCELL
Conv900 Div Div Conv I
A A B 8
Fig 4 Large-scale features of the Indian Summer Monsoon
Above flow patterns in lower and upper troposphere Below cross-circulation along B - B (entrance)
and A - A (exit region of the Tropical Easterly Jet)
- 18 shy
S50 roh level Apparently lifting and consequently rainfall
reach a maximum in their southern section there is hardly
sufficient evidence of definite air-mass differences in their
area Generally speaking air-mass differences during the
monsoon season above the subcontinent are much smaller than
in middle latitudes and hardly detectable in the lower
troposphere However in the upper troposphere we observe a remarkable baroclinie structure where the equatorial air
above SON is about 7-10oC cooler than the subtropical air
above the heat eentre at 28-30oN This reversed temperature
gradient is eorrelated with an extremely persistent tropiealo
easterly jet (FLOHN 1964) centred near latitude IS N one or two km below the tropieal tropopause This easterly jet is
of remarkable persistence from day to day with only oecasional fluetuations of the wind velocity (pulses) whieh
have a duration of three to six days and varying between
about 40 and more than 100 knots in the centre According to
recent studies by RAMAN (1964) these pulses are eorrelated
with fluctuations of the rainfall above the subcontinent and thus with the varying release of latent heat which
intensifies the anticyelonie eells above the Himalayas
On the average the speed of the Tropical Easterly Jet inshy
creases (together with a northerly component of the winds)
east of 850 E and decreases (together with a weak southerly
eomponent) towards the west This causes (FLOHN 1964) a eross-isobaric meridional circulation reverting its sign
near 8SoE (Figure 4) Above SE Asia ascending motion with
strong cloudiness and rainfall prevails along the northern edge of the system (Lat 20-32oN) while subsiding motions
are frequently found along th~ southern edge near the equator Above SW-Asia and Africathe reverse is true (FLOHN
1964) large-scale subsidence tendency along the northern flane (including the area of the Intertropical Convergence
Zone at the surface) in contrast to lifting motions along
~
- 19 shy
the southern edge (the belt of highest rainfall along Lat
7-14oN)
Nevertheless climatic features near surfaee are by no means
unimportant in the large-scale meehanism of the summer-monsoon
rains in southern Asia We list only a few of them
a) The reversal of the temperature pressure and wind field
of the middle and upper troposphere is preceded by the gradual
warming of northern India the Tibetan highlands and the
adjaeent highlands Bere we have to distinguish two main
sources of internal and potential energy
1 The flux of sensible heat into the air from the elevated
heat souree of the arid (western and central) seetion of
the Tibetan highlands Here all terms of the heat balance
can be assumed to be nearly equal to those in the adjacent
arid lowlands of centra1 Asia Suffieient evidence for
this assumption has been presented by AIZENSHTAT (1966)
from many heat balance investigations in the arid high
mountains of the USSR at altitudes of 3100-4000 m The
average input of sensible heat into the atmosphere amounts
to 180-250 Lyday (1 Ly = 1 calcm2)
2 The release of latent heat from the enormous quantities
of rain falling on the superhumid mountain areas of Bengal
Assam and Upper Burma This has an estimated area-averaged value for the cloud layer of 900-1200 Lyd ie more
than the global radiation at the surfaee can yield Its
effect has been suggested generally (FLOHN 1953) as an
addition to the anticyclonic curvature of the upper westershy
lies along high mountain ridges Oue to the rapid decrease
of moisture with height it will increase the thiekness of
the lower troposphere however the aseending motion in the
eb towers carries much heat upward to the 150-300 mb layer
- 20 shy - 21 shy
Most of this latent heat is used for heating up the upper
troposphere to a moist-adiabatic lapse-rate Since in SEshy
Tibet and Assam the average vertical lapse-rate is slightly
super-moist-adiabatie any ascending parcel of air in the
eondensation layer between 775 and 205 mb is about 16oC
warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in
the arid areas
The effect of the direct warming of air above southernmost
and western Tibet and north-eastern India has been qualitashy
tively stressed by the author (1950 1960 1968) Strong
evidenee has been contributed in a model eomputation by
MURAKAMI (1958) who started from the average 300 mb-flow
during May introduced a heat source above the whole
Tibetan plateau and obtained after a model time of 72
hours an antieyelonic eell in southeastern Tibet In a
more recent model of a two-dimensional meridional cross-
section along BOoE the role of the mountains and of the
hydrologie cycle has been investigated numerically by the
same author (1969) Using correlation analysis ASAKURA
(196B) recently demonstrated the strong persistence and
the large-seale influence of the Tibetan anticyelone at
the 500 mb-level
Careful analysis of all available aerological data shy
taking Into account the diurnal variation of upper air
temperatures as well as thermaI winds (FLOHN 196B) shy
revealed beyond any doubt the existenee of an elongated
heat zone in the upper troposphere (300-500 mb) above the
Himalayas and southern Tibet centered at 300 N and 850S
(Figure 5)
r e
N
~ I
2 I
~
abull
+ lt lJl middotri
t 0
Q) ~ III U Ul
Ul Cl U t 0
-rl ~
-1 Q) III gtlt
III ~ ~ Q)
c + 0
Q)Cl t nl l +
nl (l) (l)
gtltIII ~
-1 Q) +
~ t rtI Q)0
0 S If)
I 0 0 + 0 c M -rl
Ul t Ul Q) Q) t + 10( + U lt1l - p c
+ Il Ul Ul Q) + t 104
10( Ql Q ~ c 0
u If)
lJl r
- 22 shy
b) In the arid north-western part of the subcontinent (Sind
Rajasthan) the aridity is maintained even during the monsoon
season when moist air with an almost constant dewpoint near 0
25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days
when monsoon depressions travelling from the east or south
penetrate this area On all other days - ie during more
than 90 per cent of the season - the permanent and steady
monsoon flow is divergent turning from south-west above
Jodhpur and Gwalior to south-south-east above Jacobabad and
therefore subject to large-seale subsidence (FLOHN et al
196B) which reduces the vertical extension of the moist
layer to about 15-2 km and its eloud cover to shallow
stratocumuli (sc) This divergence can be understood if we
take into aceount the average position of the pressure trough
with its anticyelonic curvature from eastern Arabia to the
Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1
Similarly the SW-monsoon above the Arabian Sea is - at least
west of Long 65-68o
E - essentially rainless with a shallow
layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by
directional diffluence but by the increasing velocity along
the stream lines About 500-BOO kms off the Indian west coast
the wind speed decreases accompanied by a remarkable increase
of shower activity These facts strongly support our present
view of the dynamic effects of the wind vergences in contrast
to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)
cl The remarkable Foumlhn effect on the eastern side of the
West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a
frequency of 29-30 rain-days a month from June to September)
we observe only 20-40 km east of the crest not more than
- 23 shy
600-800 mm mostly produced by a few travelling synoptic
disturbances In some of the deepest valleys of the Himalaya
Mountains - even in the rainiest areas north of Assam - the
valley bottom is almost arid (as demonstrated by the vegeshy
tation) (SCHWEINFURTH 1956) since the local circulations
always control the pattern of wind components clouds and
rain (FLOHN 1970 DITTMANN 1970) This is also true in the
deep meridional gorges of Upper Burma - in spite of the high
cloudiness and moisture content of the air during late spring
and summer below the quasi-permanent anticyclonic cell in the
upper troposphere The slight anticyclonic curvature of the
lower westerlies crossing the Ghats (Figure 2) might also be
related to the permanent release of latent heat
d) The distribution of land- and sea-breezes controls a large
part of the convective activity The frequency and extend of
these diurnal circulations along all the coasts of the subshy
continent deserves much more attention together with their
effect on the weather During the summer monsoon these
diurnal circulations are to a large extend weakened or even
suppressed During the remaining seasons the quasi-geostrophic
flow near the surface is much weaker the diurnal radiation
cycle is more effective due to low cloudiness and the diurnal
circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)
middote) Careful analysis of the diurnal wind circulation at the
Himalayas led to the unexpected conclusion that the usual
reversal from the ascending day-time circulation to the
descending nighttime circulation is to a large extend
suppressed by the permanence of the above-mentioned heat
center in the upper troposphere during night (FLOHN 196B
1970) Thus in many valleys in the Himalayas (including the
meridional gorges) the weak nighttime mountain winds are
either superimposed with upstream winds or are replaced by calms
- 24 shy
D The retreat of the monsoon during September and October
(REITER amp HEUBERGER 1960) seems not to be merely a mirror
of the events during the onset in early June The large-scale
reversamiddotl of the wind field is somewhat more gradual and
relatively slow but low-Ievel cyclones still cross the
peninsula and may be steered in a northward direction causing
veritable cloudbursts and severe floods Due to the gradual
~~ifting of their tracks they often have the opportunity to
ltevelop over ocean areas into full-size warm-core hurricanes
notably over the Gulf of Bengal From October to December
tropical hurricanes and other cyclones of minor intensity
are fairly frequent here they can almost certainly be tracked
along their path from the east ie from the Gulf of Thailand
They are responsible for the late fall rains on the east coast
(and over the southcentral part) of the peninsula as weIl as
for the rainy season on all eastfacing coasts of south-east
Asia In contrast to spring they are in this season relatively
rare (but not absent) over the Arabian Sea Since the northshy
east flow itself is shallow and mostly stable at least north
of about latitude 12oN its effect on rainfall is small The
fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations
from year to year Since the cyclonic disturbances usually
travel towards west the shearing intensity reaches its maxishy
mum at their western side Together with the increase of
surface stress at the east-facing coasts this leads to a
marked rainfall maximum along these coasts
E In order to avoid further misunderstanding it should be
stressed that the monsoonal reversal of the surface winds is
by no means identical with begin and end of the rainy season
In fact the rainy season starts in the northeastern area of
the subcontinent about two months earlier than the reversal
of (upper) winds while in the heart of the peninsula the
- 25 shy
low-level wind shift precedes the onset of the monsoon rains
by about the same time
The early onset of rains in NE-India contributes - together
with the seasonal warming of the elevated Tibetan highlands shy
to the development of the high-tropospheric summer anticyclones
above the Himalayas This event produces the reversal of the
winds of the mlddle and upper troposphere which now triggers
the onset of the monsoon rains above the largest portion of
the subcontinent Such an interaction between thermodynamic
and dynamic processes is by no means uncommon in the largeshy
scale atmospheric circulation In this area it is not merely
a seasonal shift but an orographically fixed reversal of
temperature and pressure gradients which leads to such dramatic
(and far-reaching) consequences Before the existence of the
Tibetan highlands - ie before the late-Tertiary (and recent)
uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa
or Australia This is also evident from the recent model
computation of MURAKAMI (1969)
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
- 8 shy
A) During winter - especially from mid-December to February shy
upper westerlies prevail at the maximum wind level near 200 mb
above the whole area north of latitude ISoN and in all layers
above the Punjab and northernmost India They may even reach
the southern tip of the peninsula Below the westerlies the
well-known north-east monsoon - whieh mayaiso correct1y be
defined as the north-east trade - is quite shallow (1 - 15 km)
in the latitudinal belt between 220 and 300 N its vertical
depth increases towards the south above the peninsula Oue to
the Foumlhn effeet of the surrounding high mountains this
north-east monsoon eonsists of dry eontinental air diverging
and subsiding as in most trade wind areas
Within the upper westerlies the subtropical jet ean be obshy
served nearly every day between latitude 250 and 30o
N with
frequent troughs and ridges (Rossby waves) moving fair1y
rapidly in an easterly direetion from North Africa to southern
China (KRISHNAMURTI 1961 RAMAGE 1952) In the advanee
seetion of the troughs where vertical stretching prevails
together with upper divergenee and lower eonvergenee
(RAMASWAMY 1956) we observe not_ infrequently strong eonshy
veetive activity whieh is indieated at first by Ae cast or
Ae floe then by large towering eumulonimbus (with strong
shear in the forerunning anvil) Due to the extremely dry
air with eumulus bases frequently situated at 4 or 5 km
a large part of the falling rain evaporates before reaching
the ground but heavy gusts thunder and lightning and in
arid regions dust or sand storms are not rare events On
the warm southern side of the subtropieal jet preeipitation
eonneeted with these systems is in most eases only weak In
the north-west of the subeontinent espeeially in northern
Punjab and Baluchistan and in the Himalayas and their footshy
hills these Christmas rains (equivalent to the Mediterranean
winter rains) form adefinite rainy season varying in intensity
largely from year to year These rains are also responsible
- 9 shy
for the high snow cover of the Hindukush of the Karakorum
and of the north-western Himalayas (FLOHN 1969) These high
precipitations are by no means reflected in the measurements
of valley stations like Leh or Gilgit here the regular
occurence of daytime upslope breezes diverging from the
valley bottom prevents substantial precipitation (SCHWEINFURTH
1956) Only on rare oeeasions do these winter rains extend
into the Indian peninsula but then may reach as far as lSoN
The weak orographie eurvature of the Himalayas seems to be
responsible for the frequent occurrenee of troughs near 900 E
(Gulf of Benga1) where the average wind direction shifts
from 2S0-2900 in the west to about 260-2700 in the east The
ro1e of the Himalayas during this season can be interpreted
as a persistent cold source due to the eooling power of the
elevated snow-cover in the layer 3 - 7 km (or 700 - 400 mbs)+
Together with the warm subtropieal low-lands of northern India
this cold source forms a strong baroelinic zone probably
together with a weak direct meridional ce1l with rising warm
air and subsiding cool air In the northern Punjab - as weIl
+ According to satellite pictures (ESSA 3 and 6 January 1967
and FebruaryjMareh 1965) the eoverage of permanent snow
in the Himalayas east of 780 E is usually only 30-50 percent
in contrast to Karakorum and Hindukush mountains with more
than 70 percent Large areas of southern Tibet - especially
between 7SoE and 920 E and south of 300 N - are snow-free on
about SO percent of all days The other 20 of the days
travelling disturbances produce clouds and snow which
disappears rapidly due to strong daytime radiation and
evaporation Even centra1 Tibet appears to be snow-free in
about 50 percent of all cases (FLOHN 1968) In contrast to
the marginal mountains the highlands of southern and cen~ral
Tibet ean act as an elevated heat source even during winter
- 10 shy
as in adjacent Afghanistan and Baluchistan _ the winter rains
increase in intensity (and sometimes also in frequency six to
ten days a month) during March and April but decrease during May
The above-mentioned direct meridional cell is not reflected
in the precipitation distribution In fact th~ average preshy
cipitation in the mountains is much greater due to orography
than in the plains Recent investigations on the water balance of the Hindukush and adjacent mountains have shown that the
winter-spring snowfall in the mountains is much greater than
hitherto estimated (FLOHN 1969) This confirrns other studies
on the mass balance of the enormous glaciers of the Karakorurn the Pamirs etc
B) During spring the distribution of the upper westerlies
is nearly the same as during winter including the existence
of the quasi-permanent Bengal trough But the increasing solar
radiation leads to the development of shallow but persistent
heat lews above the southern part of the peninsula which
gradually move to the north and eventually reach (in early
June) the Punjab On their southern side shallow and weak
westerlies - which during the last century were falsely
called sea-breezes - occur with little or no effect on
weather Over the Bay of Bengal and across its northern coast
weak southerly (SW-SE) winds with moist maritime air gradushy
ally replace the dry north-east monsoon Now the divergence
aloft in front of a travelling upper trough of the subtropical jet frequently coincides above north-east India (Bengal
Assam East Pakistan and perhaps also Upper Burma) with a
shallow layer of unstable moist air a situation which
typically produces severe storms Therefore from mid-March
to May the squalls of the Bengal Nor-westers increase
greatly in frequency intensity and rainfall With this
- 11 shy
synoptic mechanism the rainy season of north-eastern India
starts during spring much earlier than the marked reversal
in the upper winds At many places the nurnber of rainy days
(with 25 rnrn and more) increases to 12-19 per month due to
the orographical trapping of the low-level southerly flow in
the Assam area which is surrounded on three sides by high
mountains During April and May (FtoHN 1968) 10 typical synshy
optic stations in the Assam plains receive as much as 24 rnrn
per rainy day the whole area receives as much as 88 rnrn per
day regardless of the nurnber of stations with rain and in
only 7 percent of all cases is the area rainless (The same
nurnbers for northern Germany during summer are 50 rnrn 30 rnrn
and 10 percent respectively)
Over the Indian Ocean some tropical cyclones are observed
most of which can be followed from the South China Sea or
the Gulf of Thailand as they progress to the eoast Occasioshy
nally they develop into full-size tropical hurricanes with
warm eores and with anticyclonic flows above 200 rnb They are
then steered toward the north-west or even north in the
advance section of a westerly trough and are accompanied by
torrential rains and floods Despite their rareness they
may even re-oceur after a week or so and are sometimes
accompanied by disastrous flood effects in the densely popushy
lated low-level area in the delta of the Ganges and Brahrnashy
putra
Cl During May and early June the seasollal warming of the air
above NE-India - produeed by the release of latent heat from
precipitation processes - and above southern Tibet - here
caused by the elevation of the heated surface near 4500 m
(equivalent to about 580 rnb) - weakens and finally reverses
the meridional gradient of temperature between subtropical
latitudes and the equator (FLOHN 1964) According to
- 12 shy
aero1ogieal data this proeess starts from the uppermost troposphere (150-200 mb) and proeeeds slow1y downwards The
reversal of this meridional temperature gradient in the 1ayer 800-500 mb oecurs almost simu1taneous1y with the onset of monaoon rains along the western eoast of the peninsula This results - almost simultaneously with the northward displaeement of the low-level heat low - in a complete
reversal of the upper winds ( between about 450 and 100 mb)
from west to east a rapid inerease of the vertical depth of the equatorial westerlies now known as the south-west monsoon to 6-7 km and their intrusion into the low-pressure area extending from the Punjab to Assam and further to the eastshy
south-east This syndrome of nearly simu1taneous events is observed under normal conditions during the first ten days
of June in some years it oeeurs already during late May in others it may be postponed unti1 late June It is wel1-known
in classical textbooks as the burst of the monsoon But after recognizing the lack of coincidence between rainfall
distribution thunder-storm frequency and three-dimensiona1 wind patterns we see that it is actually quite a diffieult job to define exaet1y one (and only one) date for this event
at a given point The literal translation of the Arabic term maus im is quite generally season therefore this term has originally no specifie meaning like monsoon winds or monsoon rains The lack of coincidenee between monsoon winds
and monsoon rains is responsible for frequent misinterpreshy
tations and confusion the onset of the rainymiddot season is eontrolled in different areas by quite different synoptic
features
This reversal of the three-dimensional wind field 1a aceompanied or better triggered by the occurrence of anticyclonic cells in the divergence zone between the high-tropospheric eastershylies and the westerlies near latitude 30oN The average
position of the subtropical jet shifts to about 40oN with a
- 13 shy
weak anticyelonic curvature on the northern fringe of the Tibetan highlands This rapid displacement coincides with a re-arrangement of the quasi-permanent troughs (Figure 1)
Instead of the weak cyclonie eurvature of the subtropical jet near 900 E (Bengal trough) we now observe two rather
marked troughs just upstream and downstream of the Tibetan block the Pamir trough (near 650 E) and the west Chinese (or
Szechwan) trough (at lOO-10SoB)
We might remark at this point without further explanations that nearly simultaneously the quasi-permanent secondary trough over eastern Europe (centred around 300 E) disappears
together with a rapid decrease of the number of b10cking anticyclones above north-western Europe a consequence of
Rossbys well-known stationary wave formula This causes a marked singularity of weather in large parts of Europe
unfortunately misnamed the European summer monaoon Furthershy
more at this time the occurrence of a quasi-stationary antishycyclonic cell above the cool Okhotsk Sea together with the
sharpening of the Szechwan trough is responsible for the occurrence of Bai-U - rains above central and northern
China and Japan (ASAKURA 1968) which are mainly produced by weak cyclones running along the marked baroclinic frontal zone (Pacific Polar Front) Wh1le these two events are
certainly related in a dynamical sense to the events between Pamir and Szechwan this 1s probably not the case with respect
to a third event the onset of Arizona summer rains which apparently have many parallels to the simultaneous developshyments in the Punjab These large scale teleconnections deserve a comparative investigation
The usual distinction between an_Arabian branch and a Bengal branch of the south-west monsoonis somewhat misleading In
both areas westerly winds of themiddot lower troposphere - mostly
250-2900 - converge along a slowiy displacing convergenoe
_____ _
- 14 shy
4~ m1c-1
~_--15
10 0-000 SIIt1f11lf bullbullbullbull Co~1 S a~orl I bull
bull I Sbtntpual _1 bullbull is lJlff tnhl9h AXIS
Fig 1 Position of the Fig 3 Meridional crossshysubtropical and of the section of zonal winds semi-permanent troughs along Long 780 E (arrows = during summer and during upper air stations) Julyshythe cool seasons August 1961-62 (October-May) bull
Fig 2 Position of the monsoon pressure trough (= northern
branch of the ITC) and streamlines of the resultant
wind at 09 kms during summer
- l c shy
line with a deep current from east-south-east (Figure 2)
which extends right up to the easterlies of the upper troposhy
sphere At the southern edge of this convergence - which forms
part of the main ITCZ (northern branch) and the equatorial
pressure trough - isolated cyclonic disturbances without
distinct fronts and air-masses frequently travel from eastshy
south-east to west-north-west producing much rain in their
southern and western sections These monsoon depressions
are responsible for the bulk of the summer rains in the
interior of the lndo-Pakistan subcontinent Their forecast
depends mainly on sufficient knowledge of the rather comshy
plicated three-dimensional wind field in that area up to at
least 200 mb Not infrequently these disturbances can be
followed during their earlier track from the east Apart
from these low-level cyclones frequent strong convective
rains are observed in two regions in Bengal and Assam where
the low-tropospheric south-west flow permanently converges
with the easterlies (including orographically foreed lifting
at the Khasia Hills) and at the steep escarpments of the
western eoasts of both South-Asian peninsulae But even along
these eoastal escarpments large variations from day to day
oeeur probably related to variations of the divergenee of
low-level winds The strong contribution of eonvective
activity- interrupted by short rainless subsidence periods shy
to the summer rain in Nepal has been demonstrated by DITTMANN
(1970)
On the average the boundary between the extra-tropical
westerlies and the easterlies above the south-west monsoon
(Figure 3) is situated between 400-100 mb (7-16 km) near
300 N ie just above a large part of the Himalaya mountains
At the same time the subtropical jet is situated at latitude
40-420 N ie above the Tarim Basin as revealed by Chinese
aerological data with a rernarkable constaney The boundary
between these currents is marked by well-defined anticyclonic
- 16 shy
cells frequently centered in the south-eastern portion of
the Tibetan highlands Sometimes the travelling upper troughs of the westerlies may penetrate near 200 mb to about latishy
tude 26-27oN but hardly ever farther south If~ in such a case a low-level monsoon depression approaches from southshy
east it can be intensified and steered into a northerly
direction and eventually merge with the trough Then torrential
rains floods and landslides are observed in the Himalayas
while the divergence of the west-south-west flow above the
peninsula produces widespread subsidence and suppression of
convective activity This is at least one of the mechanisms
responsible for the well-known monsoon breaks (RAHMATULLAH
1952 RIEHL 1954) In fact if we disregard the relatively
small areas of orographically forced convective rains and of
permanent convergence in the north-eastern portion of the
subcontinent we may safely say that ove~ the largest part
of the Indo-Pakistan area the summer rains are produced by
well-differentiated synoptic events and cannot be conceived
as a more or less steady climatic feature Even the quasishy
permanent anticyclonic cell above south-eastern Tibet (FLOHN 1958 1968) can be swept away by a travelling westerly trough
with a cold air outbreak across Tibet it recovers however
after two to three days sometimes being reinforced by warm
air advection Thererore maps of rainfall frequency (SCHWEINshyFURTH et al 1970) are a necessary implementation of the
well-known maps of precipitation amount
The track of these monsoon depressions is controlled by the
upper flow or bette~ by integration of the wind field
horizontally and vertically with respect to pressure and
taking into account the well-known meridional variation of
the Coriolis parameter szlig = dfdy Under normal conditions
they are therefore steered from east-south-east to west-northshywest only gradually diminishing in intensity and rainfall
productivity mostly as fairly symmetrie vortices near the
- 17 shy
o0 I I Irltt I 60 0 110deg
Flow Patterns 800-900mbs-+
100 - 300 IIE-
Cold Warm Co Id Warm100 I mbs INDIRECT
500 CELL GEClCELL
Conv900 Div Div Conv I
A A B 8
Fig 4 Large-scale features of the Indian Summer Monsoon
Above flow patterns in lower and upper troposphere Below cross-circulation along B - B (entrance)
and A - A (exit region of the Tropical Easterly Jet)
- 18 shy
S50 roh level Apparently lifting and consequently rainfall
reach a maximum in their southern section there is hardly
sufficient evidence of definite air-mass differences in their
area Generally speaking air-mass differences during the
monsoon season above the subcontinent are much smaller than
in middle latitudes and hardly detectable in the lower
troposphere However in the upper troposphere we observe a remarkable baroclinie structure where the equatorial air
above SON is about 7-10oC cooler than the subtropical air
above the heat eentre at 28-30oN This reversed temperature
gradient is eorrelated with an extremely persistent tropiealo
easterly jet (FLOHN 1964) centred near latitude IS N one or two km below the tropieal tropopause This easterly jet is
of remarkable persistence from day to day with only oecasional fluetuations of the wind velocity (pulses) whieh
have a duration of three to six days and varying between
about 40 and more than 100 knots in the centre According to
recent studies by RAMAN (1964) these pulses are eorrelated
with fluctuations of the rainfall above the subcontinent and thus with the varying release of latent heat which
intensifies the anticyelonie eells above the Himalayas
On the average the speed of the Tropical Easterly Jet inshy
creases (together with a northerly component of the winds)
east of 850 E and decreases (together with a weak southerly
eomponent) towards the west This causes (FLOHN 1964) a eross-isobaric meridional circulation reverting its sign
near 8SoE (Figure 4) Above SE Asia ascending motion with
strong cloudiness and rainfall prevails along the northern edge of the system (Lat 20-32oN) while subsiding motions
are frequently found along th~ southern edge near the equator Above SW-Asia and Africathe reverse is true (FLOHN
1964) large-scale subsidence tendency along the northern flane (including the area of the Intertropical Convergence
Zone at the surface) in contrast to lifting motions along
~
- 19 shy
the southern edge (the belt of highest rainfall along Lat
7-14oN)
Nevertheless climatic features near surfaee are by no means
unimportant in the large-scale meehanism of the summer-monsoon
rains in southern Asia We list only a few of them
a) The reversal of the temperature pressure and wind field
of the middle and upper troposphere is preceded by the gradual
warming of northern India the Tibetan highlands and the
adjaeent highlands Bere we have to distinguish two main
sources of internal and potential energy
1 The flux of sensible heat into the air from the elevated
heat souree of the arid (western and central) seetion of
the Tibetan highlands Here all terms of the heat balance
can be assumed to be nearly equal to those in the adjacent
arid lowlands of centra1 Asia Suffieient evidence for
this assumption has been presented by AIZENSHTAT (1966)
from many heat balance investigations in the arid high
mountains of the USSR at altitudes of 3100-4000 m The
average input of sensible heat into the atmosphere amounts
to 180-250 Lyday (1 Ly = 1 calcm2)
2 The release of latent heat from the enormous quantities
of rain falling on the superhumid mountain areas of Bengal
Assam and Upper Burma This has an estimated area-averaged value for the cloud layer of 900-1200 Lyd ie more
than the global radiation at the surfaee can yield Its
effect has been suggested generally (FLOHN 1953) as an
addition to the anticyclonic curvature of the upper westershy
lies along high mountain ridges Oue to the rapid decrease
of moisture with height it will increase the thiekness of
the lower troposphere however the aseending motion in the
eb towers carries much heat upward to the 150-300 mb layer
- 20 shy - 21 shy
Most of this latent heat is used for heating up the upper
troposphere to a moist-adiabatic lapse-rate Since in SEshy
Tibet and Assam the average vertical lapse-rate is slightly
super-moist-adiabatie any ascending parcel of air in the
eondensation layer between 775 and 205 mb is about 16oC
warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in
the arid areas
The effect of the direct warming of air above southernmost
and western Tibet and north-eastern India has been qualitashy
tively stressed by the author (1950 1960 1968) Strong
evidenee has been contributed in a model eomputation by
MURAKAMI (1958) who started from the average 300 mb-flow
during May introduced a heat source above the whole
Tibetan plateau and obtained after a model time of 72
hours an antieyelonic eell in southeastern Tibet In a
more recent model of a two-dimensional meridional cross-
section along BOoE the role of the mountains and of the
hydrologie cycle has been investigated numerically by the
same author (1969) Using correlation analysis ASAKURA
(196B) recently demonstrated the strong persistence and
the large-seale influence of the Tibetan anticyelone at
the 500 mb-level
Careful analysis of all available aerological data shy
taking Into account the diurnal variation of upper air
temperatures as well as thermaI winds (FLOHN 196B) shy
revealed beyond any doubt the existenee of an elongated
heat zone in the upper troposphere (300-500 mb) above the
Himalayas and southern Tibet centered at 300 N and 850S
(Figure 5)
r e
N
~ I
2 I
~
abull
+ lt lJl middotri
t 0
Q) ~ III U Ul
Ul Cl U t 0
-rl ~
-1 Q) III gtlt
III ~ ~ Q)
c + 0
Q)Cl t nl l +
nl (l) (l)
gtltIII ~
-1 Q) +
~ t rtI Q)0
0 S If)
I 0 0 + 0 c M -rl
Ul t Ul Q) Q) t + 10( + U lt1l - p c
+ Il Ul Ul Q) + t 104
10( Ql Q ~ c 0
u If)
lJl r
- 22 shy
b) In the arid north-western part of the subcontinent (Sind
Rajasthan) the aridity is maintained even during the monsoon
season when moist air with an almost constant dewpoint near 0
25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days
when monsoon depressions travelling from the east or south
penetrate this area On all other days - ie during more
than 90 per cent of the season - the permanent and steady
monsoon flow is divergent turning from south-west above
Jodhpur and Gwalior to south-south-east above Jacobabad and
therefore subject to large-seale subsidence (FLOHN et al
196B) which reduces the vertical extension of the moist
layer to about 15-2 km and its eloud cover to shallow
stratocumuli (sc) This divergence can be understood if we
take into aceount the average position of the pressure trough
with its anticyelonic curvature from eastern Arabia to the
Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1
Similarly the SW-monsoon above the Arabian Sea is - at least
west of Long 65-68o
E - essentially rainless with a shallow
layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by
directional diffluence but by the increasing velocity along
the stream lines About 500-BOO kms off the Indian west coast
the wind speed decreases accompanied by a remarkable increase
of shower activity These facts strongly support our present
view of the dynamic effects of the wind vergences in contrast
to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)
cl The remarkable Foumlhn effect on the eastern side of the
West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a
frequency of 29-30 rain-days a month from June to September)
we observe only 20-40 km east of the crest not more than
- 23 shy
600-800 mm mostly produced by a few travelling synoptic
disturbances In some of the deepest valleys of the Himalaya
Mountains - even in the rainiest areas north of Assam - the
valley bottom is almost arid (as demonstrated by the vegeshy
tation) (SCHWEINFURTH 1956) since the local circulations
always control the pattern of wind components clouds and
rain (FLOHN 1970 DITTMANN 1970) This is also true in the
deep meridional gorges of Upper Burma - in spite of the high
cloudiness and moisture content of the air during late spring
and summer below the quasi-permanent anticyclonic cell in the
upper troposphere The slight anticyclonic curvature of the
lower westerlies crossing the Ghats (Figure 2) might also be
related to the permanent release of latent heat
d) The distribution of land- and sea-breezes controls a large
part of the convective activity The frequency and extend of
these diurnal circulations along all the coasts of the subshy
continent deserves much more attention together with their
effect on the weather During the summer monsoon these
diurnal circulations are to a large extend weakened or even
suppressed During the remaining seasons the quasi-geostrophic
flow near the surface is much weaker the diurnal radiation
cycle is more effective due to low cloudiness and the diurnal
circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)
middote) Careful analysis of the diurnal wind circulation at the
Himalayas led to the unexpected conclusion that the usual
reversal from the ascending day-time circulation to the
descending nighttime circulation is to a large extend
suppressed by the permanence of the above-mentioned heat
center in the upper troposphere during night (FLOHN 196B
1970) Thus in many valleys in the Himalayas (including the
meridional gorges) the weak nighttime mountain winds are
either superimposed with upstream winds or are replaced by calms
- 24 shy
D The retreat of the monsoon during September and October
(REITER amp HEUBERGER 1960) seems not to be merely a mirror
of the events during the onset in early June The large-scale
reversamiddotl of the wind field is somewhat more gradual and
relatively slow but low-Ievel cyclones still cross the
peninsula and may be steered in a northward direction causing
veritable cloudbursts and severe floods Due to the gradual
~~ifting of their tracks they often have the opportunity to
ltevelop over ocean areas into full-size warm-core hurricanes
notably over the Gulf of Bengal From October to December
tropical hurricanes and other cyclones of minor intensity
are fairly frequent here they can almost certainly be tracked
along their path from the east ie from the Gulf of Thailand
They are responsible for the late fall rains on the east coast
(and over the southcentral part) of the peninsula as weIl as
for the rainy season on all eastfacing coasts of south-east
Asia In contrast to spring they are in this season relatively
rare (but not absent) over the Arabian Sea Since the northshy
east flow itself is shallow and mostly stable at least north
of about latitude 12oN its effect on rainfall is small The
fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations
from year to year Since the cyclonic disturbances usually
travel towards west the shearing intensity reaches its maxishy
mum at their western side Together with the increase of
surface stress at the east-facing coasts this leads to a
marked rainfall maximum along these coasts
E In order to avoid further misunderstanding it should be
stressed that the monsoonal reversal of the surface winds is
by no means identical with begin and end of the rainy season
In fact the rainy season starts in the northeastern area of
the subcontinent about two months earlier than the reversal
of (upper) winds while in the heart of the peninsula the
- 25 shy
low-level wind shift precedes the onset of the monsoon rains
by about the same time
The early onset of rains in NE-India contributes - together
with the seasonal warming of the elevated Tibetan highlands shy
to the development of the high-tropospheric summer anticyclones
above the Himalayas This event produces the reversal of the
winds of the mlddle and upper troposphere which now triggers
the onset of the monsoon rains above the largest portion of
the subcontinent Such an interaction between thermodynamic
and dynamic processes is by no means uncommon in the largeshy
scale atmospheric circulation In this area it is not merely
a seasonal shift but an orographically fixed reversal of
temperature and pressure gradients which leads to such dramatic
(and far-reaching) consequences Before the existence of the
Tibetan highlands - ie before the late-Tertiary (and recent)
uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa
or Australia This is also evident from the recent model
computation of MURAKAMI (1969)
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
- 10 shy
as in adjacent Afghanistan and Baluchistan _ the winter rains
increase in intensity (and sometimes also in frequency six to
ten days a month) during March and April but decrease during May
The above-mentioned direct meridional cell is not reflected
in the precipitation distribution In fact th~ average preshy
cipitation in the mountains is much greater due to orography
than in the plains Recent investigations on the water balance of the Hindukush and adjacent mountains have shown that the
winter-spring snowfall in the mountains is much greater than
hitherto estimated (FLOHN 1969) This confirrns other studies
on the mass balance of the enormous glaciers of the Karakorurn the Pamirs etc
B) During spring the distribution of the upper westerlies
is nearly the same as during winter including the existence
of the quasi-permanent Bengal trough But the increasing solar
radiation leads to the development of shallow but persistent
heat lews above the southern part of the peninsula which
gradually move to the north and eventually reach (in early
June) the Punjab On their southern side shallow and weak
westerlies - which during the last century were falsely
called sea-breezes - occur with little or no effect on
weather Over the Bay of Bengal and across its northern coast
weak southerly (SW-SE) winds with moist maritime air gradushy
ally replace the dry north-east monsoon Now the divergence
aloft in front of a travelling upper trough of the subtropical jet frequently coincides above north-east India (Bengal
Assam East Pakistan and perhaps also Upper Burma) with a
shallow layer of unstable moist air a situation which
typically produces severe storms Therefore from mid-March
to May the squalls of the Bengal Nor-westers increase
greatly in frequency intensity and rainfall With this
- 11 shy
synoptic mechanism the rainy season of north-eastern India
starts during spring much earlier than the marked reversal
in the upper winds At many places the nurnber of rainy days
(with 25 rnrn and more) increases to 12-19 per month due to
the orographical trapping of the low-level southerly flow in
the Assam area which is surrounded on three sides by high
mountains During April and May (FtoHN 1968) 10 typical synshy
optic stations in the Assam plains receive as much as 24 rnrn
per rainy day the whole area receives as much as 88 rnrn per
day regardless of the nurnber of stations with rain and in
only 7 percent of all cases is the area rainless (The same
nurnbers for northern Germany during summer are 50 rnrn 30 rnrn
and 10 percent respectively)
Over the Indian Ocean some tropical cyclones are observed
most of which can be followed from the South China Sea or
the Gulf of Thailand as they progress to the eoast Occasioshy
nally they develop into full-size tropical hurricanes with
warm eores and with anticyclonic flows above 200 rnb They are
then steered toward the north-west or even north in the
advance section of a westerly trough and are accompanied by
torrential rains and floods Despite their rareness they
may even re-oceur after a week or so and are sometimes
accompanied by disastrous flood effects in the densely popushy
lated low-level area in the delta of the Ganges and Brahrnashy
putra
Cl During May and early June the seasollal warming of the air
above NE-India - produeed by the release of latent heat from
precipitation processes - and above southern Tibet - here
caused by the elevation of the heated surface near 4500 m
(equivalent to about 580 rnb) - weakens and finally reverses
the meridional gradient of temperature between subtropical
latitudes and the equator (FLOHN 1964) According to
- 12 shy
aero1ogieal data this proeess starts from the uppermost troposphere (150-200 mb) and proeeeds slow1y downwards The
reversal of this meridional temperature gradient in the 1ayer 800-500 mb oecurs almost simu1taneous1y with the onset of monaoon rains along the western eoast of the peninsula This results - almost simultaneously with the northward displaeement of the low-level heat low - in a complete
reversal of the upper winds ( between about 450 and 100 mb)
from west to east a rapid inerease of the vertical depth of the equatorial westerlies now known as the south-west monsoon to 6-7 km and their intrusion into the low-pressure area extending from the Punjab to Assam and further to the eastshy
south-east This syndrome of nearly simu1taneous events is observed under normal conditions during the first ten days
of June in some years it oeeurs already during late May in others it may be postponed unti1 late June It is wel1-known
in classical textbooks as the burst of the monsoon But after recognizing the lack of coincidence between rainfall
distribution thunder-storm frequency and three-dimensiona1 wind patterns we see that it is actually quite a diffieult job to define exaet1y one (and only one) date for this event
at a given point The literal translation of the Arabic term maus im is quite generally season therefore this term has originally no specifie meaning like monsoon winds or monsoon rains The lack of coincidenee between monsoon winds
and monsoon rains is responsible for frequent misinterpreshy
tations and confusion the onset of the rainymiddot season is eontrolled in different areas by quite different synoptic
features
This reversal of the three-dimensional wind field 1a aceompanied or better triggered by the occurrence of anticyclonic cells in the divergence zone between the high-tropospheric eastershylies and the westerlies near latitude 30oN The average
position of the subtropical jet shifts to about 40oN with a
- 13 shy
weak anticyelonic curvature on the northern fringe of the Tibetan highlands This rapid displacement coincides with a re-arrangement of the quasi-permanent troughs (Figure 1)
Instead of the weak cyclonie eurvature of the subtropical jet near 900 E (Bengal trough) we now observe two rather
marked troughs just upstream and downstream of the Tibetan block the Pamir trough (near 650 E) and the west Chinese (or
Szechwan) trough (at lOO-10SoB)
We might remark at this point without further explanations that nearly simultaneously the quasi-permanent secondary trough over eastern Europe (centred around 300 E) disappears
together with a rapid decrease of the number of b10cking anticyclones above north-western Europe a consequence of
Rossbys well-known stationary wave formula This causes a marked singularity of weather in large parts of Europe
unfortunately misnamed the European summer monaoon Furthershy
more at this time the occurrence of a quasi-stationary antishycyclonic cell above the cool Okhotsk Sea together with the
sharpening of the Szechwan trough is responsible for the occurrence of Bai-U - rains above central and northern
China and Japan (ASAKURA 1968) which are mainly produced by weak cyclones running along the marked baroclinic frontal zone (Pacific Polar Front) Wh1le these two events are
certainly related in a dynamical sense to the events between Pamir and Szechwan this 1s probably not the case with respect
to a third event the onset of Arizona summer rains which apparently have many parallels to the simultaneous developshyments in the Punjab These large scale teleconnections deserve a comparative investigation
The usual distinction between an_Arabian branch and a Bengal branch of the south-west monsoonis somewhat misleading In
both areas westerly winds of themiddot lower troposphere - mostly
250-2900 - converge along a slowiy displacing convergenoe
_____ _
- 14 shy
4~ m1c-1
~_--15
10 0-000 SIIt1f11lf bullbullbullbull Co~1 S a~orl I bull
bull I Sbtntpual _1 bullbull is lJlff tnhl9h AXIS
Fig 1 Position of the Fig 3 Meridional crossshysubtropical and of the section of zonal winds semi-permanent troughs along Long 780 E (arrows = during summer and during upper air stations) Julyshythe cool seasons August 1961-62 (October-May) bull
Fig 2 Position of the monsoon pressure trough (= northern
branch of the ITC) and streamlines of the resultant
wind at 09 kms during summer
- l c shy
line with a deep current from east-south-east (Figure 2)
which extends right up to the easterlies of the upper troposhy
sphere At the southern edge of this convergence - which forms
part of the main ITCZ (northern branch) and the equatorial
pressure trough - isolated cyclonic disturbances without
distinct fronts and air-masses frequently travel from eastshy
south-east to west-north-west producing much rain in their
southern and western sections These monsoon depressions
are responsible for the bulk of the summer rains in the
interior of the lndo-Pakistan subcontinent Their forecast
depends mainly on sufficient knowledge of the rather comshy
plicated three-dimensional wind field in that area up to at
least 200 mb Not infrequently these disturbances can be
followed during their earlier track from the east Apart
from these low-level cyclones frequent strong convective
rains are observed in two regions in Bengal and Assam where
the low-tropospheric south-west flow permanently converges
with the easterlies (including orographically foreed lifting
at the Khasia Hills) and at the steep escarpments of the
western eoasts of both South-Asian peninsulae But even along
these eoastal escarpments large variations from day to day
oeeur probably related to variations of the divergenee of
low-level winds The strong contribution of eonvective
activity- interrupted by short rainless subsidence periods shy
to the summer rain in Nepal has been demonstrated by DITTMANN
(1970)
On the average the boundary between the extra-tropical
westerlies and the easterlies above the south-west monsoon
(Figure 3) is situated between 400-100 mb (7-16 km) near
300 N ie just above a large part of the Himalaya mountains
At the same time the subtropical jet is situated at latitude
40-420 N ie above the Tarim Basin as revealed by Chinese
aerological data with a rernarkable constaney The boundary
between these currents is marked by well-defined anticyclonic
- 16 shy
cells frequently centered in the south-eastern portion of
the Tibetan highlands Sometimes the travelling upper troughs of the westerlies may penetrate near 200 mb to about latishy
tude 26-27oN but hardly ever farther south If~ in such a case a low-level monsoon depression approaches from southshy
east it can be intensified and steered into a northerly
direction and eventually merge with the trough Then torrential
rains floods and landslides are observed in the Himalayas
while the divergence of the west-south-west flow above the
peninsula produces widespread subsidence and suppression of
convective activity This is at least one of the mechanisms
responsible for the well-known monsoon breaks (RAHMATULLAH
1952 RIEHL 1954) In fact if we disregard the relatively
small areas of orographically forced convective rains and of
permanent convergence in the north-eastern portion of the
subcontinent we may safely say that ove~ the largest part
of the Indo-Pakistan area the summer rains are produced by
well-differentiated synoptic events and cannot be conceived
as a more or less steady climatic feature Even the quasishy
permanent anticyclonic cell above south-eastern Tibet (FLOHN 1958 1968) can be swept away by a travelling westerly trough
with a cold air outbreak across Tibet it recovers however
after two to three days sometimes being reinforced by warm
air advection Thererore maps of rainfall frequency (SCHWEINshyFURTH et al 1970) are a necessary implementation of the
well-known maps of precipitation amount
The track of these monsoon depressions is controlled by the
upper flow or bette~ by integration of the wind field
horizontally and vertically with respect to pressure and
taking into account the well-known meridional variation of
the Coriolis parameter szlig = dfdy Under normal conditions
they are therefore steered from east-south-east to west-northshywest only gradually diminishing in intensity and rainfall
productivity mostly as fairly symmetrie vortices near the
- 17 shy
o0 I I Irltt I 60 0 110deg
Flow Patterns 800-900mbs-+
100 - 300 IIE-
Cold Warm Co Id Warm100 I mbs INDIRECT
500 CELL GEClCELL
Conv900 Div Div Conv I
A A B 8
Fig 4 Large-scale features of the Indian Summer Monsoon
Above flow patterns in lower and upper troposphere Below cross-circulation along B - B (entrance)
and A - A (exit region of the Tropical Easterly Jet)
- 18 shy
S50 roh level Apparently lifting and consequently rainfall
reach a maximum in their southern section there is hardly
sufficient evidence of definite air-mass differences in their
area Generally speaking air-mass differences during the
monsoon season above the subcontinent are much smaller than
in middle latitudes and hardly detectable in the lower
troposphere However in the upper troposphere we observe a remarkable baroclinie structure where the equatorial air
above SON is about 7-10oC cooler than the subtropical air
above the heat eentre at 28-30oN This reversed temperature
gradient is eorrelated with an extremely persistent tropiealo
easterly jet (FLOHN 1964) centred near latitude IS N one or two km below the tropieal tropopause This easterly jet is
of remarkable persistence from day to day with only oecasional fluetuations of the wind velocity (pulses) whieh
have a duration of three to six days and varying between
about 40 and more than 100 knots in the centre According to
recent studies by RAMAN (1964) these pulses are eorrelated
with fluctuations of the rainfall above the subcontinent and thus with the varying release of latent heat which
intensifies the anticyelonie eells above the Himalayas
On the average the speed of the Tropical Easterly Jet inshy
creases (together with a northerly component of the winds)
east of 850 E and decreases (together with a weak southerly
eomponent) towards the west This causes (FLOHN 1964) a eross-isobaric meridional circulation reverting its sign
near 8SoE (Figure 4) Above SE Asia ascending motion with
strong cloudiness and rainfall prevails along the northern edge of the system (Lat 20-32oN) while subsiding motions
are frequently found along th~ southern edge near the equator Above SW-Asia and Africathe reverse is true (FLOHN
1964) large-scale subsidence tendency along the northern flane (including the area of the Intertropical Convergence
Zone at the surface) in contrast to lifting motions along
~
- 19 shy
the southern edge (the belt of highest rainfall along Lat
7-14oN)
Nevertheless climatic features near surfaee are by no means
unimportant in the large-scale meehanism of the summer-monsoon
rains in southern Asia We list only a few of them
a) The reversal of the temperature pressure and wind field
of the middle and upper troposphere is preceded by the gradual
warming of northern India the Tibetan highlands and the
adjaeent highlands Bere we have to distinguish two main
sources of internal and potential energy
1 The flux of sensible heat into the air from the elevated
heat souree of the arid (western and central) seetion of
the Tibetan highlands Here all terms of the heat balance
can be assumed to be nearly equal to those in the adjacent
arid lowlands of centra1 Asia Suffieient evidence for
this assumption has been presented by AIZENSHTAT (1966)
from many heat balance investigations in the arid high
mountains of the USSR at altitudes of 3100-4000 m The
average input of sensible heat into the atmosphere amounts
to 180-250 Lyday (1 Ly = 1 calcm2)
2 The release of latent heat from the enormous quantities
of rain falling on the superhumid mountain areas of Bengal
Assam and Upper Burma This has an estimated area-averaged value for the cloud layer of 900-1200 Lyd ie more
than the global radiation at the surfaee can yield Its
effect has been suggested generally (FLOHN 1953) as an
addition to the anticyclonic curvature of the upper westershy
lies along high mountain ridges Oue to the rapid decrease
of moisture with height it will increase the thiekness of
the lower troposphere however the aseending motion in the
eb towers carries much heat upward to the 150-300 mb layer
- 20 shy - 21 shy
Most of this latent heat is used for heating up the upper
troposphere to a moist-adiabatic lapse-rate Since in SEshy
Tibet and Assam the average vertical lapse-rate is slightly
super-moist-adiabatie any ascending parcel of air in the
eondensation layer between 775 and 205 mb is about 16oC
warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in
the arid areas
The effect of the direct warming of air above southernmost
and western Tibet and north-eastern India has been qualitashy
tively stressed by the author (1950 1960 1968) Strong
evidenee has been contributed in a model eomputation by
MURAKAMI (1958) who started from the average 300 mb-flow
during May introduced a heat source above the whole
Tibetan plateau and obtained after a model time of 72
hours an antieyelonic eell in southeastern Tibet In a
more recent model of a two-dimensional meridional cross-
section along BOoE the role of the mountains and of the
hydrologie cycle has been investigated numerically by the
same author (1969) Using correlation analysis ASAKURA
(196B) recently demonstrated the strong persistence and
the large-seale influence of the Tibetan anticyelone at
the 500 mb-level
Careful analysis of all available aerological data shy
taking Into account the diurnal variation of upper air
temperatures as well as thermaI winds (FLOHN 196B) shy
revealed beyond any doubt the existenee of an elongated
heat zone in the upper troposphere (300-500 mb) above the
Himalayas and southern Tibet centered at 300 N and 850S
(Figure 5)
r e
N
~ I
2 I
~
abull
+ lt lJl middotri
t 0
Q) ~ III U Ul
Ul Cl U t 0
-rl ~
-1 Q) III gtlt
III ~ ~ Q)
c + 0
Q)Cl t nl l +
nl (l) (l)
gtltIII ~
-1 Q) +
~ t rtI Q)0
0 S If)
I 0 0 + 0 c M -rl
Ul t Ul Q) Q) t + 10( + U lt1l - p c
+ Il Ul Ul Q) + t 104
10( Ql Q ~ c 0
u If)
lJl r
- 22 shy
b) In the arid north-western part of the subcontinent (Sind
Rajasthan) the aridity is maintained even during the monsoon
season when moist air with an almost constant dewpoint near 0
25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days
when monsoon depressions travelling from the east or south
penetrate this area On all other days - ie during more
than 90 per cent of the season - the permanent and steady
monsoon flow is divergent turning from south-west above
Jodhpur and Gwalior to south-south-east above Jacobabad and
therefore subject to large-seale subsidence (FLOHN et al
196B) which reduces the vertical extension of the moist
layer to about 15-2 km and its eloud cover to shallow
stratocumuli (sc) This divergence can be understood if we
take into aceount the average position of the pressure trough
with its anticyelonic curvature from eastern Arabia to the
Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1
Similarly the SW-monsoon above the Arabian Sea is - at least
west of Long 65-68o
E - essentially rainless with a shallow
layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by
directional diffluence but by the increasing velocity along
the stream lines About 500-BOO kms off the Indian west coast
the wind speed decreases accompanied by a remarkable increase
of shower activity These facts strongly support our present
view of the dynamic effects of the wind vergences in contrast
to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)
cl The remarkable Foumlhn effect on the eastern side of the
West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a
frequency of 29-30 rain-days a month from June to September)
we observe only 20-40 km east of the crest not more than
- 23 shy
600-800 mm mostly produced by a few travelling synoptic
disturbances In some of the deepest valleys of the Himalaya
Mountains - even in the rainiest areas north of Assam - the
valley bottom is almost arid (as demonstrated by the vegeshy
tation) (SCHWEINFURTH 1956) since the local circulations
always control the pattern of wind components clouds and
rain (FLOHN 1970 DITTMANN 1970) This is also true in the
deep meridional gorges of Upper Burma - in spite of the high
cloudiness and moisture content of the air during late spring
and summer below the quasi-permanent anticyclonic cell in the
upper troposphere The slight anticyclonic curvature of the
lower westerlies crossing the Ghats (Figure 2) might also be
related to the permanent release of latent heat
d) The distribution of land- and sea-breezes controls a large
part of the convective activity The frequency and extend of
these diurnal circulations along all the coasts of the subshy
continent deserves much more attention together with their
effect on the weather During the summer monsoon these
diurnal circulations are to a large extend weakened or even
suppressed During the remaining seasons the quasi-geostrophic
flow near the surface is much weaker the diurnal radiation
cycle is more effective due to low cloudiness and the diurnal
circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)
middote) Careful analysis of the diurnal wind circulation at the
Himalayas led to the unexpected conclusion that the usual
reversal from the ascending day-time circulation to the
descending nighttime circulation is to a large extend
suppressed by the permanence of the above-mentioned heat
center in the upper troposphere during night (FLOHN 196B
1970) Thus in many valleys in the Himalayas (including the
meridional gorges) the weak nighttime mountain winds are
either superimposed with upstream winds or are replaced by calms
- 24 shy
D The retreat of the monsoon during September and October
(REITER amp HEUBERGER 1960) seems not to be merely a mirror
of the events during the onset in early June The large-scale
reversamiddotl of the wind field is somewhat more gradual and
relatively slow but low-Ievel cyclones still cross the
peninsula and may be steered in a northward direction causing
veritable cloudbursts and severe floods Due to the gradual
~~ifting of their tracks they often have the opportunity to
ltevelop over ocean areas into full-size warm-core hurricanes
notably over the Gulf of Bengal From October to December
tropical hurricanes and other cyclones of minor intensity
are fairly frequent here they can almost certainly be tracked
along their path from the east ie from the Gulf of Thailand
They are responsible for the late fall rains on the east coast
(and over the southcentral part) of the peninsula as weIl as
for the rainy season on all eastfacing coasts of south-east
Asia In contrast to spring they are in this season relatively
rare (but not absent) over the Arabian Sea Since the northshy
east flow itself is shallow and mostly stable at least north
of about latitude 12oN its effect on rainfall is small The
fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations
from year to year Since the cyclonic disturbances usually
travel towards west the shearing intensity reaches its maxishy
mum at their western side Together with the increase of
surface stress at the east-facing coasts this leads to a
marked rainfall maximum along these coasts
E In order to avoid further misunderstanding it should be
stressed that the monsoonal reversal of the surface winds is
by no means identical with begin and end of the rainy season
In fact the rainy season starts in the northeastern area of
the subcontinent about two months earlier than the reversal
of (upper) winds while in the heart of the peninsula the
- 25 shy
low-level wind shift precedes the onset of the monsoon rains
by about the same time
The early onset of rains in NE-India contributes - together
with the seasonal warming of the elevated Tibetan highlands shy
to the development of the high-tropospheric summer anticyclones
above the Himalayas This event produces the reversal of the
winds of the mlddle and upper troposphere which now triggers
the onset of the monsoon rains above the largest portion of
the subcontinent Such an interaction between thermodynamic
and dynamic processes is by no means uncommon in the largeshy
scale atmospheric circulation In this area it is not merely
a seasonal shift but an orographically fixed reversal of
temperature and pressure gradients which leads to such dramatic
(and far-reaching) consequences Before the existence of the
Tibetan highlands - ie before the late-Tertiary (and recent)
uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa
or Australia This is also evident from the recent model
computation of MURAKAMI (1969)
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
- 12 shy
aero1ogieal data this proeess starts from the uppermost troposphere (150-200 mb) and proeeeds slow1y downwards The
reversal of this meridional temperature gradient in the 1ayer 800-500 mb oecurs almost simu1taneous1y with the onset of monaoon rains along the western eoast of the peninsula This results - almost simultaneously with the northward displaeement of the low-level heat low - in a complete
reversal of the upper winds ( between about 450 and 100 mb)
from west to east a rapid inerease of the vertical depth of the equatorial westerlies now known as the south-west monsoon to 6-7 km and their intrusion into the low-pressure area extending from the Punjab to Assam and further to the eastshy
south-east This syndrome of nearly simu1taneous events is observed under normal conditions during the first ten days
of June in some years it oeeurs already during late May in others it may be postponed unti1 late June It is wel1-known
in classical textbooks as the burst of the monsoon But after recognizing the lack of coincidence between rainfall
distribution thunder-storm frequency and three-dimensiona1 wind patterns we see that it is actually quite a diffieult job to define exaet1y one (and only one) date for this event
at a given point The literal translation of the Arabic term maus im is quite generally season therefore this term has originally no specifie meaning like monsoon winds or monsoon rains The lack of coincidenee between monsoon winds
and monsoon rains is responsible for frequent misinterpreshy
tations and confusion the onset of the rainymiddot season is eontrolled in different areas by quite different synoptic
features
This reversal of the three-dimensional wind field 1a aceompanied or better triggered by the occurrence of anticyclonic cells in the divergence zone between the high-tropospheric eastershylies and the westerlies near latitude 30oN The average
position of the subtropical jet shifts to about 40oN with a
- 13 shy
weak anticyelonic curvature on the northern fringe of the Tibetan highlands This rapid displacement coincides with a re-arrangement of the quasi-permanent troughs (Figure 1)
Instead of the weak cyclonie eurvature of the subtropical jet near 900 E (Bengal trough) we now observe two rather
marked troughs just upstream and downstream of the Tibetan block the Pamir trough (near 650 E) and the west Chinese (or
Szechwan) trough (at lOO-10SoB)
We might remark at this point without further explanations that nearly simultaneously the quasi-permanent secondary trough over eastern Europe (centred around 300 E) disappears
together with a rapid decrease of the number of b10cking anticyclones above north-western Europe a consequence of
Rossbys well-known stationary wave formula This causes a marked singularity of weather in large parts of Europe
unfortunately misnamed the European summer monaoon Furthershy
more at this time the occurrence of a quasi-stationary antishycyclonic cell above the cool Okhotsk Sea together with the
sharpening of the Szechwan trough is responsible for the occurrence of Bai-U - rains above central and northern
China and Japan (ASAKURA 1968) which are mainly produced by weak cyclones running along the marked baroclinic frontal zone (Pacific Polar Front) Wh1le these two events are
certainly related in a dynamical sense to the events between Pamir and Szechwan this 1s probably not the case with respect
to a third event the onset of Arizona summer rains which apparently have many parallels to the simultaneous developshyments in the Punjab These large scale teleconnections deserve a comparative investigation
The usual distinction between an_Arabian branch and a Bengal branch of the south-west monsoonis somewhat misleading In
both areas westerly winds of themiddot lower troposphere - mostly
250-2900 - converge along a slowiy displacing convergenoe
_____ _
- 14 shy
4~ m1c-1
~_--15
10 0-000 SIIt1f11lf bullbullbullbull Co~1 S a~orl I bull
bull I Sbtntpual _1 bullbull is lJlff tnhl9h AXIS
Fig 1 Position of the Fig 3 Meridional crossshysubtropical and of the section of zonal winds semi-permanent troughs along Long 780 E (arrows = during summer and during upper air stations) Julyshythe cool seasons August 1961-62 (October-May) bull
Fig 2 Position of the monsoon pressure trough (= northern
branch of the ITC) and streamlines of the resultant
wind at 09 kms during summer
- l c shy
line with a deep current from east-south-east (Figure 2)
which extends right up to the easterlies of the upper troposhy
sphere At the southern edge of this convergence - which forms
part of the main ITCZ (northern branch) and the equatorial
pressure trough - isolated cyclonic disturbances without
distinct fronts and air-masses frequently travel from eastshy
south-east to west-north-west producing much rain in their
southern and western sections These monsoon depressions
are responsible for the bulk of the summer rains in the
interior of the lndo-Pakistan subcontinent Their forecast
depends mainly on sufficient knowledge of the rather comshy
plicated three-dimensional wind field in that area up to at
least 200 mb Not infrequently these disturbances can be
followed during their earlier track from the east Apart
from these low-level cyclones frequent strong convective
rains are observed in two regions in Bengal and Assam where
the low-tropospheric south-west flow permanently converges
with the easterlies (including orographically foreed lifting
at the Khasia Hills) and at the steep escarpments of the
western eoasts of both South-Asian peninsulae But even along
these eoastal escarpments large variations from day to day
oeeur probably related to variations of the divergenee of
low-level winds The strong contribution of eonvective
activity- interrupted by short rainless subsidence periods shy
to the summer rain in Nepal has been demonstrated by DITTMANN
(1970)
On the average the boundary between the extra-tropical
westerlies and the easterlies above the south-west monsoon
(Figure 3) is situated between 400-100 mb (7-16 km) near
300 N ie just above a large part of the Himalaya mountains
At the same time the subtropical jet is situated at latitude
40-420 N ie above the Tarim Basin as revealed by Chinese
aerological data with a rernarkable constaney The boundary
between these currents is marked by well-defined anticyclonic
- 16 shy
cells frequently centered in the south-eastern portion of
the Tibetan highlands Sometimes the travelling upper troughs of the westerlies may penetrate near 200 mb to about latishy
tude 26-27oN but hardly ever farther south If~ in such a case a low-level monsoon depression approaches from southshy
east it can be intensified and steered into a northerly
direction and eventually merge with the trough Then torrential
rains floods and landslides are observed in the Himalayas
while the divergence of the west-south-west flow above the
peninsula produces widespread subsidence and suppression of
convective activity This is at least one of the mechanisms
responsible for the well-known monsoon breaks (RAHMATULLAH
1952 RIEHL 1954) In fact if we disregard the relatively
small areas of orographically forced convective rains and of
permanent convergence in the north-eastern portion of the
subcontinent we may safely say that ove~ the largest part
of the Indo-Pakistan area the summer rains are produced by
well-differentiated synoptic events and cannot be conceived
as a more or less steady climatic feature Even the quasishy
permanent anticyclonic cell above south-eastern Tibet (FLOHN 1958 1968) can be swept away by a travelling westerly trough
with a cold air outbreak across Tibet it recovers however
after two to three days sometimes being reinforced by warm
air advection Thererore maps of rainfall frequency (SCHWEINshyFURTH et al 1970) are a necessary implementation of the
well-known maps of precipitation amount
The track of these monsoon depressions is controlled by the
upper flow or bette~ by integration of the wind field
horizontally and vertically with respect to pressure and
taking into account the well-known meridional variation of
the Coriolis parameter szlig = dfdy Under normal conditions
they are therefore steered from east-south-east to west-northshywest only gradually diminishing in intensity and rainfall
productivity mostly as fairly symmetrie vortices near the
- 17 shy
o0 I I Irltt I 60 0 110deg
Flow Patterns 800-900mbs-+
100 - 300 IIE-
Cold Warm Co Id Warm100 I mbs INDIRECT
500 CELL GEClCELL
Conv900 Div Div Conv I
A A B 8
Fig 4 Large-scale features of the Indian Summer Monsoon
Above flow patterns in lower and upper troposphere Below cross-circulation along B - B (entrance)
and A - A (exit region of the Tropical Easterly Jet)
- 18 shy
S50 roh level Apparently lifting and consequently rainfall
reach a maximum in their southern section there is hardly
sufficient evidence of definite air-mass differences in their
area Generally speaking air-mass differences during the
monsoon season above the subcontinent are much smaller than
in middle latitudes and hardly detectable in the lower
troposphere However in the upper troposphere we observe a remarkable baroclinie structure where the equatorial air
above SON is about 7-10oC cooler than the subtropical air
above the heat eentre at 28-30oN This reversed temperature
gradient is eorrelated with an extremely persistent tropiealo
easterly jet (FLOHN 1964) centred near latitude IS N one or two km below the tropieal tropopause This easterly jet is
of remarkable persistence from day to day with only oecasional fluetuations of the wind velocity (pulses) whieh
have a duration of three to six days and varying between
about 40 and more than 100 knots in the centre According to
recent studies by RAMAN (1964) these pulses are eorrelated
with fluctuations of the rainfall above the subcontinent and thus with the varying release of latent heat which
intensifies the anticyelonie eells above the Himalayas
On the average the speed of the Tropical Easterly Jet inshy
creases (together with a northerly component of the winds)
east of 850 E and decreases (together with a weak southerly
eomponent) towards the west This causes (FLOHN 1964) a eross-isobaric meridional circulation reverting its sign
near 8SoE (Figure 4) Above SE Asia ascending motion with
strong cloudiness and rainfall prevails along the northern edge of the system (Lat 20-32oN) while subsiding motions
are frequently found along th~ southern edge near the equator Above SW-Asia and Africathe reverse is true (FLOHN
1964) large-scale subsidence tendency along the northern flane (including the area of the Intertropical Convergence
Zone at the surface) in contrast to lifting motions along
~
- 19 shy
the southern edge (the belt of highest rainfall along Lat
7-14oN)
Nevertheless climatic features near surfaee are by no means
unimportant in the large-scale meehanism of the summer-monsoon
rains in southern Asia We list only a few of them
a) The reversal of the temperature pressure and wind field
of the middle and upper troposphere is preceded by the gradual
warming of northern India the Tibetan highlands and the
adjaeent highlands Bere we have to distinguish two main
sources of internal and potential energy
1 The flux of sensible heat into the air from the elevated
heat souree of the arid (western and central) seetion of
the Tibetan highlands Here all terms of the heat balance
can be assumed to be nearly equal to those in the adjacent
arid lowlands of centra1 Asia Suffieient evidence for
this assumption has been presented by AIZENSHTAT (1966)
from many heat balance investigations in the arid high
mountains of the USSR at altitudes of 3100-4000 m The
average input of sensible heat into the atmosphere amounts
to 180-250 Lyday (1 Ly = 1 calcm2)
2 The release of latent heat from the enormous quantities
of rain falling on the superhumid mountain areas of Bengal
Assam and Upper Burma This has an estimated area-averaged value for the cloud layer of 900-1200 Lyd ie more
than the global radiation at the surfaee can yield Its
effect has been suggested generally (FLOHN 1953) as an
addition to the anticyclonic curvature of the upper westershy
lies along high mountain ridges Oue to the rapid decrease
of moisture with height it will increase the thiekness of
the lower troposphere however the aseending motion in the
eb towers carries much heat upward to the 150-300 mb layer
- 20 shy - 21 shy
Most of this latent heat is used for heating up the upper
troposphere to a moist-adiabatic lapse-rate Since in SEshy
Tibet and Assam the average vertical lapse-rate is slightly
super-moist-adiabatie any ascending parcel of air in the
eondensation layer between 775 and 205 mb is about 16oC
warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in
the arid areas
The effect of the direct warming of air above southernmost
and western Tibet and north-eastern India has been qualitashy
tively stressed by the author (1950 1960 1968) Strong
evidenee has been contributed in a model eomputation by
MURAKAMI (1958) who started from the average 300 mb-flow
during May introduced a heat source above the whole
Tibetan plateau and obtained after a model time of 72
hours an antieyelonic eell in southeastern Tibet In a
more recent model of a two-dimensional meridional cross-
section along BOoE the role of the mountains and of the
hydrologie cycle has been investigated numerically by the
same author (1969) Using correlation analysis ASAKURA
(196B) recently demonstrated the strong persistence and
the large-seale influence of the Tibetan anticyelone at
the 500 mb-level
Careful analysis of all available aerological data shy
taking Into account the diurnal variation of upper air
temperatures as well as thermaI winds (FLOHN 196B) shy
revealed beyond any doubt the existenee of an elongated
heat zone in the upper troposphere (300-500 mb) above the
Himalayas and southern Tibet centered at 300 N and 850S
(Figure 5)
r e
N
~ I
2 I
~
abull
+ lt lJl middotri
t 0
Q) ~ III U Ul
Ul Cl U t 0
-rl ~
-1 Q) III gtlt
III ~ ~ Q)
c + 0
Q)Cl t nl l +
nl (l) (l)
gtltIII ~
-1 Q) +
~ t rtI Q)0
0 S If)
I 0 0 + 0 c M -rl
Ul t Ul Q) Q) t + 10( + U lt1l - p c
+ Il Ul Ul Q) + t 104
10( Ql Q ~ c 0
u If)
lJl r
- 22 shy
b) In the arid north-western part of the subcontinent (Sind
Rajasthan) the aridity is maintained even during the monsoon
season when moist air with an almost constant dewpoint near 0
25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days
when monsoon depressions travelling from the east or south
penetrate this area On all other days - ie during more
than 90 per cent of the season - the permanent and steady
monsoon flow is divergent turning from south-west above
Jodhpur and Gwalior to south-south-east above Jacobabad and
therefore subject to large-seale subsidence (FLOHN et al
196B) which reduces the vertical extension of the moist
layer to about 15-2 km and its eloud cover to shallow
stratocumuli (sc) This divergence can be understood if we
take into aceount the average position of the pressure trough
with its anticyelonic curvature from eastern Arabia to the
Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1
Similarly the SW-monsoon above the Arabian Sea is - at least
west of Long 65-68o
E - essentially rainless with a shallow
layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by
directional diffluence but by the increasing velocity along
the stream lines About 500-BOO kms off the Indian west coast
the wind speed decreases accompanied by a remarkable increase
of shower activity These facts strongly support our present
view of the dynamic effects of the wind vergences in contrast
to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)
cl The remarkable Foumlhn effect on the eastern side of the
West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a
frequency of 29-30 rain-days a month from June to September)
we observe only 20-40 km east of the crest not more than
- 23 shy
600-800 mm mostly produced by a few travelling synoptic
disturbances In some of the deepest valleys of the Himalaya
Mountains - even in the rainiest areas north of Assam - the
valley bottom is almost arid (as demonstrated by the vegeshy
tation) (SCHWEINFURTH 1956) since the local circulations
always control the pattern of wind components clouds and
rain (FLOHN 1970 DITTMANN 1970) This is also true in the
deep meridional gorges of Upper Burma - in spite of the high
cloudiness and moisture content of the air during late spring
and summer below the quasi-permanent anticyclonic cell in the
upper troposphere The slight anticyclonic curvature of the
lower westerlies crossing the Ghats (Figure 2) might also be
related to the permanent release of latent heat
d) The distribution of land- and sea-breezes controls a large
part of the convective activity The frequency and extend of
these diurnal circulations along all the coasts of the subshy
continent deserves much more attention together with their
effect on the weather During the summer monsoon these
diurnal circulations are to a large extend weakened or even
suppressed During the remaining seasons the quasi-geostrophic
flow near the surface is much weaker the diurnal radiation
cycle is more effective due to low cloudiness and the diurnal
circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)
middote) Careful analysis of the diurnal wind circulation at the
Himalayas led to the unexpected conclusion that the usual
reversal from the ascending day-time circulation to the
descending nighttime circulation is to a large extend
suppressed by the permanence of the above-mentioned heat
center in the upper troposphere during night (FLOHN 196B
1970) Thus in many valleys in the Himalayas (including the
meridional gorges) the weak nighttime mountain winds are
either superimposed with upstream winds or are replaced by calms
- 24 shy
D The retreat of the monsoon during September and October
(REITER amp HEUBERGER 1960) seems not to be merely a mirror
of the events during the onset in early June The large-scale
reversamiddotl of the wind field is somewhat more gradual and
relatively slow but low-Ievel cyclones still cross the
peninsula and may be steered in a northward direction causing
veritable cloudbursts and severe floods Due to the gradual
~~ifting of their tracks they often have the opportunity to
ltevelop over ocean areas into full-size warm-core hurricanes
notably over the Gulf of Bengal From October to December
tropical hurricanes and other cyclones of minor intensity
are fairly frequent here they can almost certainly be tracked
along their path from the east ie from the Gulf of Thailand
They are responsible for the late fall rains on the east coast
(and over the southcentral part) of the peninsula as weIl as
for the rainy season on all eastfacing coasts of south-east
Asia In contrast to spring they are in this season relatively
rare (but not absent) over the Arabian Sea Since the northshy
east flow itself is shallow and mostly stable at least north
of about latitude 12oN its effect on rainfall is small The
fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations
from year to year Since the cyclonic disturbances usually
travel towards west the shearing intensity reaches its maxishy
mum at their western side Together with the increase of
surface stress at the east-facing coasts this leads to a
marked rainfall maximum along these coasts
E In order to avoid further misunderstanding it should be
stressed that the monsoonal reversal of the surface winds is
by no means identical with begin and end of the rainy season
In fact the rainy season starts in the northeastern area of
the subcontinent about two months earlier than the reversal
of (upper) winds while in the heart of the peninsula the
- 25 shy
low-level wind shift precedes the onset of the monsoon rains
by about the same time
The early onset of rains in NE-India contributes - together
with the seasonal warming of the elevated Tibetan highlands shy
to the development of the high-tropospheric summer anticyclones
above the Himalayas This event produces the reversal of the
winds of the mlddle and upper troposphere which now triggers
the onset of the monsoon rains above the largest portion of
the subcontinent Such an interaction between thermodynamic
and dynamic processes is by no means uncommon in the largeshy
scale atmospheric circulation In this area it is not merely
a seasonal shift but an orographically fixed reversal of
temperature and pressure gradients which leads to such dramatic
(and far-reaching) consequences Before the existence of the
Tibetan highlands - ie before the late-Tertiary (and recent)
uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa
or Australia This is also evident from the recent model
computation of MURAKAMI (1969)
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
_____ _
- 14 shy
4~ m1c-1
~_--15
10 0-000 SIIt1f11lf bullbullbullbull Co~1 S a~orl I bull
bull I Sbtntpual _1 bullbull is lJlff tnhl9h AXIS
Fig 1 Position of the Fig 3 Meridional crossshysubtropical and of the section of zonal winds semi-permanent troughs along Long 780 E (arrows = during summer and during upper air stations) Julyshythe cool seasons August 1961-62 (October-May) bull
Fig 2 Position of the monsoon pressure trough (= northern
branch of the ITC) and streamlines of the resultant
wind at 09 kms during summer
- l c shy
line with a deep current from east-south-east (Figure 2)
which extends right up to the easterlies of the upper troposhy
sphere At the southern edge of this convergence - which forms
part of the main ITCZ (northern branch) and the equatorial
pressure trough - isolated cyclonic disturbances without
distinct fronts and air-masses frequently travel from eastshy
south-east to west-north-west producing much rain in their
southern and western sections These monsoon depressions
are responsible for the bulk of the summer rains in the
interior of the lndo-Pakistan subcontinent Their forecast
depends mainly on sufficient knowledge of the rather comshy
plicated three-dimensional wind field in that area up to at
least 200 mb Not infrequently these disturbances can be
followed during their earlier track from the east Apart
from these low-level cyclones frequent strong convective
rains are observed in two regions in Bengal and Assam where
the low-tropospheric south-west flow permanently converges
with the easterlies (including orographically foreed lifting
at the Khasia Hills) and at the steep escarpments of the
western eoasts of both South-Asian peninsulae But even along
these eoastal escarpments large variations from day to day
oeeur probably related to variations of the divergenee of
low-level winds The strong contribution of eonvective
activity- interrupted by short rainless subsidence periods shy
to the summer rain in Nepal has been demonstrated by DITTMANN
(1970)
On the average the boundary between the extra-tropical
westerlies and the easterlies above the south-west monsoon
(Figure 3) is situated between 400-100 mb (7-16 km) near
300 N ie just above a large part of the Himalaya mountains
At the same time the subtropical jet is situated at latitude
40-420 N ie above the Tarim Basin as revealed by Chinese
aerological data with a rernarkable constaney The boundary
between these currents is marked by well-defined anticyclonic
- 16 shy
cells frequently centered in the south-eastern portion of
the Tibetan highlands Sometimes the travelling upper troughs of the westerlies may penetrate near 200 mb to about latishy
tude 26-27oN but hardly ever farther south If~ in such a case a low-level monsoon depression approaches from southshy
east it can be intensified and steered into a northerly
direction and eventually merge with the trough Then torrential
rains floods and landslides are observed in the Himalayas
while the divergence of the west-south-west flow above the
peninsula produces widespread subsidence and suppression of
convective activity This is at least one of the mechanisms
responsible for the well-known monsoon breaks (RAHMATULLAH
1952 RIEHL 1954) In fact if we disregard the relatively
small areas of orographically forced convective rains and of
permanent convergence in the north-eastern portion of the
subcontinent we may safely say that ove~ the largest part
of the Indo-Pakistan area the summer rains are produced by
well-differentiated synoptic events and cannot be conceived
as a more or less steady climatic feature Even the quasishy
permanent anticyclonic cell above south-eastern Tibet (FLOHN 1958 1968) can be swept away by a travelling westerly trough
with a cold air outbreak across Tibet it recovers however
after two to three days sometimes being reinforced by warm
air advection Thererore maps of rainfall frequency (SCHWEINshyFURTH et al 1970) are a necessary implementation of the
well-known maps of precipitation amount
The track of these monsoon depressions is controlled by the
upper flow or bette~ by integration of the wind field
horizontally and vertically with respect to pressure and
taking into account the well-known meridional variation of
the Coriolis parameter szlig = dfdy Under normal conditions
they are therefore steered from east-south-east to west-northshywest only gradually diminishing in intensity and rainfall
productivity mostly as fairly symmetrie vortices near the
- 17 shy
o0 I I Irltt I 60 0 110deg
Flow Patterns 800-900mbs-+
100 - 300 IIE-
Cold Warm Co Id Warm100 I mbs INDIRECT
500 CELL GEClCELL
Conv900 Div Div Conv I
A A B 8
Fig 4 Large-scale features of the Indian Summer Monsoon
Above flow patterns in lower and upper troposphere Below cross-circulation along B - B (entrance)
and A - A (exit region of the Tropical Easterly Jet)
- 18 shy
S50 roh level Apparently lifting and consequently rainfall
reach a maximum in their southern section there is hardly
sufficient evidence of definite air-mass differences in their
area Generally speaking air-mass differences during the
monsoon season above the subcontinent are much smaller than
in middle latitudes and hardly detectable in the lower
troposphere However in the upper troposphere we observe a remarkable baroclinie structure where the equatorial air
above SON is about 7-10oC cooler than the subtropical air
above the heat eentre at 28-30oN This reversed temperature
gradient is eorrelated with an extremely persistent tropiealo
easterly jet (FLOHN 1964) centred near latitude IS N one or two km below the tropieal tropopause This easterly jet is
of remarkable persistence from day to day with only oecasional fluetuations of the wind velocity (pulses) whieh
have a duration of three to six days and varying between
about 40 and more than 100 knots in the centre According to
recent studies by RAMAN (1964) these pulses are eorrelated
with fluctuations of the rainfall above the subcontinent and thus with the varying release of latent heat which
intensifies the anticyelonie eells above the Himalayas
On the average the speed of the Tropical Easterly Jet inshy
creases (together with a northerly component of the winds)
east of 850 E and decreases (together with a weak southerly
eomponent) towards the west This causes (FLOHN 1964) a eross-isobaric meridional circulation reverting its sign
near 8SoE (Figure 4) Above SE Asia ascending motion with
strong cloudiness and rainfall prevails along the northern edge of the system (Lat 20-32oN) while subsiding motions
are frequently found along th~ southern edge near the equator Above SW-Asia and Africathe reverse is true (FLOHN
1964) large-scale subsidence tendency along the northern flane (including the area of the Intertropical Convergence
Zone at the surface) in contrast to lifting motions along
~
- 19 shy
the southern edge (the belt of highest rainfall along Lat
7-14oN)
Nevertheless climatic features near surfaee are by no means
unimportant in the large-scale meehanism of the summer-monsoon
rains in southern Asia We list only a few of them
a) The reversal of the temperature pressure and wind field
of the middle and upper troposphere is preceded by the gradual
warming of northern India the Tibetan highlands and the
adjaeent highlands Bere we have to distinguish two main
sources of internal and potential energy
1 The flux of sensible heat into the air from the elevated
heat souree of the arid (western and central) seetion of
the Tibetan highlands Here all terms of the heat balance
can be assumed to be nearly equal to those in the adjacent
arid lowlands of centra1 Asia Suffieient evidence for
this assumption has been presented by AIZENSHTAT (1966)
from many heat balance investigations in the arid high
mountains of the USSR at altitudes of 3100-4000 m The
average input of sensible heat into the atmosphere amounts
to 180-250 Lyday (1 Ly = 1 calcm2)
2 The release of latent heat from the enormous quantities
of rain falling on the superhumid mountain areas of Bengal
Assam and Upper Burma This has an estimated area-averaged value for the cloud layer of 900-1200 Lyd ie more
than the global radiation at the surfaee can yield Its
effect has been suggested generally (FLOHN 1953) as an
addition to the anticyclonic curvature of the upper westershy
lies along high mountain ridges Oue to the rapid decrease
of moisture with height it will increase the thiekness of
the lower troposphere however the aseending motion in the
eb towers carries much heat upward to the 150-300 mb layer
- 20 shy - 21 shy
Most of this latent heat is used for heating up the upper
troposphere to a moist-adiabatic lapse-rate Since in SEshy
Tibet and Assam the average vertical lapse-rate is slightly
super-moist-adiabatie any ascending parcel of air in the
eondensation layer between 775 and 205 mb is about 16oC
warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in
the arid areas
The effect of the direct warming of air above southernmost
and western Tibet and north-eastern India has been qualitashy
tively stressed by the author (1950 1960 1968) Strong
evidenee has been contributed in a model eomputation by
MURAKAMI (1958) who started from the average 300 mb-flow
during May introduced a heat source above the whole
Tibetan plateau and obtained after a model time of 72
hours an antieyelonic eell in southeastern Tibet In a
more recent model of a two-dimensional meridional cross-
section along BOoE the role of the mountains and of the
hydrologie cycle has been investigated numerically by the
same author (1969) Using correlation analysis ASAKURA
(196B) recently demonstrated the strong persistence and
the large-seale influence of the Tibetan anticyelone at
the 500 mb-level
Careful analysis of all available aerological data shy
taking Into account the diurnal variation of upper air
temperatures as well as thermaI winds (FLOHN 196B) shy
revealed beyond any doubt the existenee of an elongated
heat zone in the upper troposphere (300-500 mb) above the
Himalayas and southern Tibet centered at 300 N and 850S
(Figure 5)
r e
N
~ I
2 I
~
abull
+ lt lJl middotri
t 0
Q) ~ III U Ul
Ul Cl U t 0
-rl ~
-1 Q) III gtlt
III ~ ~ Q)
c + 0
Q)Cl t nl l +
nl (l) (l)
gtltIII ~
-1 Q) +
~ t rtI Q)0
0 S If)
I 0 0 + 0 c M -rl
Ul t Ul Q) Q) t + 10( + U lt1l - p c
+ Il Ul Ul Q) + t 104
10( Ql Q ~ c 0
u If)
lJl r
- 22 shy
b) In the arid north-western part of the subcontinent (Sind
Rajasthan) the aridity is maintained even during the monsoon
season when moist air with an almost constant dewpoint near 0
25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days
when monsoon depressions travelling from the east or south
penetrate this area On all other days - ie during more
than 90 per cent of the season - the permanent and steady
monsoon flow is divergent turning from south-west above
Jodhpur and Gwalior to south-south-east above Jacobabad and
therefore subject to large-seale subsidence (FLOHN et al
196B) which reduces the vertical extension of the moist
layer to about 15-2 km and its eloud cover to shallow
stratocumuli (sc) This divergence can be understood if we
take into aceount the average position of the pressure trough
with its anticyelonic curvature from eastern Arabia to the
Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1
Similarly the SW-monsoon above the Arabian Sea is - at least
west of Long 65-68o
E - essentially rainless with a shallow
layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by
directional diffluence but by the increasing velocity along
the stream lines About 500-BOO kms off the Indian west coast
the wind speed decreases accompanied by a remarkable increase
of shower activity These facts strongly support our present
view of the dynamic effects of the wind vergences in contrast
to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)
cl The remarkable Foumlhn effect on the eastern side of the
West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a
frequency of 29-30 rain-days a month from June to September)
we observe only 20-40 km east of the crest not more than
- 23 shy
600-800 mm mostly produced by a few travelling synoptic
disturbances In some of the deepest valleys of the Himalaya
Mountains - even in the rainiest areas north of Assam - the
valley bottom is almost arid (as demonstrated by the vegeshy
tation) (SCHWEINFURTH 1956) since the local circulations
always control the pattern of wind components clouds and
rain (FLOHN 1970 DITTMANN 1970) This is also true in the
deep meridional gorges of Upper Burma - in spite of the high
cloudiness and moisture content of the air during late spring
and summer below the quasi-permanent anticyclonic cell in the
upper troposphere The slight anticyclonic curvature of the
lower westerlies crossing the Ghats (Figure 2) might also be
related to the permanent release of latent heat
d) The distribution of land- and sea-breezes controls a large
part of the convective activity The frequency and extend of
these diurnal circulations along all the coasts of the subshy
continent deserves much more attention together with their
effect on the weather During the summer monsoon these
diurnal circulations are to a large extend weakened or even
suppressed During the remaining seasons the quasi-geostrophic
flow near the surface is much weaker the diurnal radiation
cycle is more effective due to low cloudiness and the diurnal
circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)
middote) Careful analysis of the diurnal wind circulation at the
Himalayas led to the unexpected conclusion that the usual
reversal from the ascending day-time circulation to the
descending nighttime circulation is to a large extend
suppressed by the permanence of the above-mentioned heat
center in the upper troposphere during night (FLOHN 196B
1970) Thus in many valleys in the Himalayas (including the
meridional gorges) the weak nighttime mountain winds are
either superimposed with upstream winds or are replaced by calms
- 24 shy
D The retreat of the monsoon during September and October
(REITER amp HEUBERGER 1960) seems not to be merely a mirror
of the events during the onset in early June The large-scale
reversamiddotl of the wind field is somewhat more gradual and
relatively slow but low-Ievel cyclones still cross the
peninsula and may be steered in a northward direction causing
veritable cloudbursts and severe floods Due to the gradual
~~ifting of their tracks they often have the opportunity to
ltevelop over ocean areas into full-size warm-core hurricanes
notably over the Gulf of Bengal From October to December
tropical hurricanes and other cyclones of minor intensity
are fairly frequent here they can almost certainly be tracked
along their path from the east ie from the Gulf of Thailand
They are responsible for the late fall rains on the east coast
(and over the southcentral part) of the peninsula as weIl as
for the rainy season on all eastfacing coasts of south-east
Asia In contrast to spring they are in this season relatively
rare (but not absent) over the Arabian Sea Since the northshy
east flow itself is shallow and mostly stable at least north
of about latitude 12oN its effect on rainfall is small The
fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations
from year to year Since the cyclonic disturbances usually
travel towards west the shearing intensity reaches its maxishy
mum at their western side Together with the increase of
surface stress at the east-facing coasts this leads to a
marked rainfall maximum along these coasts
E In order to avoid further misunderstanding it should be
stressed that the monsoonal reversal of the surface winds is
by no means identical with begin and end of the rainy season
In fact the rainy season starts in the northeastern area of
the subcontinent about two months earlier than the reversal
of (upper) winds while in the heart of the peninsula the
- 25 shy
low-level wind shift precedes the onset of the monsoon rains
by about the same time
The early onset of rains in NE-India contributes - together
with the seasonal warming of the elevated Tibetan highlands shy
to the development of the high-tropospheric summer anticyclones
above the Himalayas This event produces the reversal of the
winds of the mlddle and upper troposphere which now triggers
the onset of the monsoon rains above the largest portion of
the subcontinent Such an interaction between thermodynamic
and dynamic processes is by no means uncommon in the largeshy
scale atmospheric circulation In this area it is not merely
a seasonal shift but an orographically fixed reversal of
temperature and pressure gradients which leads to such dramatic
(and far-reaching) consequences Before the existence of the
Tibetan highlands - ie before the late-Tertiary (and recent)
uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa
or Australia This is also evident from the recent model
computation of MURAKAMI (1969)
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
- 16 shy
cells frequently centered in the south-eastern portion of
the Tibetan highlands Sometimes the travelling upper troughs of the westerlies may penetrate near 200 mb to about latishy
tude 26-27oN but hardly ever farther south If~ in such a case a low-level monsoon depression approaches from southshy
east it can be intensified and steered into a northerly
direction and eventually merge with the trough Then torrential
rains floods and landslides are observed in the Himalayas
while the divergence of the west-south-west flow above the
peninsula produces widespread subsidence and suppression of
convective activity This is at least one of the mechanisms
responsible for the well-known monsoon breaks (RAHMATULLAH
1952 RIEHL 1954) In fact if we disregard the relatively
small areas of orographically forced convective rains and of
permanent convergence in the north-eastern portion of the
subcontinent we may safely say that ove~ the largest part
of the Indo-Pakistan area the summer rains are produced by
well-differentiated synoptic events and cannot be conceived
as a more or less steady climatic feature Even the quasishy
permanent anticyclonic cell above south-eastern Tibet (FLOHN 1958 1968) can be swept away by a travelling westerly trough
with a cold air outbreak across Tibet it recovers however
after two to three days sometimes being reinforced by warm
air advection Thererore maps of rainfall frequency (SCHWEINshyFURTH et al 1970) are a necessary implementation of the
well-known maps of precipitation amount
The track of these monsoon depressions is controlled by the
upper flow or bette~ by integration of the wind field
horizontally and vertically with respect to pressure and
taking into account the well-known meridional variation of
the Coriolis parameter szlig = dfdy Under normal conditions
they are therefore steered from east-south-east to west-northshywest only gradually diminishing in intensity and rainfall
productivity mostly as fairly symmetrie vortices near the
- 17 shy
o0 I I Irltt I 60 0 110deg
Flow Patterns 800-900mbs-+
100 - 300 IIE-
Cold Warm Co Id Warm100 I mbs INDIRECT
500 CELL GEClCELL
Conv900 Div Div Conv I
A A B 8
Fig 4 Large-scale features of the Indian Summer Monsoon
Above flow patterns in lower and upper troposphere Below cross-circulation along B - B (entrance)
and A - A (exit region of the Tropical Easterly Jet)
- 18 shy
S50 roh level Apparently lifting and consequently rainfall
reach a maximum in their southern section there is hardly
sufficient evidence of definite air-mass differences in their
area Generally speaking air-mass differences during the
monsoon season above the subcontinent are much smaller than
in middle latitudes and hardly detectable in the lower
troposphere However in the upper troposphere we observe a remarkable baroclinie structure where the equatorial air
above SON is about 7-10oC cooler than the subtropical air
above the heat eentre at 28-30oN This reversed temperature
gradient is eorrelated with an extremely persistent tropiealo
easterly jet (FLOHN 1964) centred near latitude IS N one or two km below the tropieal tropopause This easterly jet is
of remarkable persistence from day to day with only oecasional fluetuations of the wind velocity (pulses) whieh
have a duration of three to six days and varying between
about 40 and more than 100 knots in the centre According to
recent studies by RAMAN (1964) these pulses are eorrelated
with fluctuations of the rainfall above the subcontinent and thus with the varying release of latent heat which
intensifies the anticyelonie eells above the Himalayas
On the average the speed of the Tropical Easterly Jet inshy
creases (together with a northerly component of the winds)
east of 850 E and decreases (together with a weak southerly
eomponent) towards the west This causes (FLOHN 1964) a eross-isobaric meridional circulation reverting its sign
near 8SoE (Figure 4) Above SE Asia ascending motion with
strong cloudiness and rainfall prevails along the northern edge of the system (Lat 20-32oN) while subsiding motions
are frequently found along th~ southern edge near the equator Above SW-Asia and Africathe reverse is true (FLOHN
1964) large-scale subsidence tendency along the northern flane (including the area of the Intertropical Convergence
Zone at the surface) in contrast to lifting motions along
~
- 19 shy
the southern edge (the belt of highest rainfall along Lat
7-14oN)
Nevertheless climatic features near surfaee are by no means
unimportant in the large-scale meehanism of the summer-monsoon
rains in southern Asia We list only a few of them
a) The reversal of the temperature pressure and wind field
of the middle and upper troposphere is preceded by the gradual
warming of northern India the Tibetan highlands and the
adjaeent highlands Bere we have to distinguish two main
sources of internal and potential energy
1 The flux of sensible heat into the air from the elevated
heat souree of the arid (western and central) seetion of
the Tibetan highlands Here all terms of the heat balance
can be assumed to be nearly equal to those in the adjacent
arid lowlands of centra1 Asia Suffieient evidence for
this assumption has been presented by AIZENSHTAT (1966)
from many heat balance investigations in the arid high
mountains of the USSR at altitudes of 3100-4000 m The
average input of sensible heat into the atmosphere amounts
to 180-250 Lyday (1 Ly = 1 calcm2)
2 The release of latent heat from the enormous quantities
of rain falling on the superhumid mountain areas of Bengal
Assam and Upper Burma This has an estimated area-averaged value for the cloud layer of 900-1200 Lyd ie more
than the global radiation at the surfaee can yield Its
effect has been suggested generally (FLOHN 1953) as an
addition to the anticyclonic curvature of the upper westershy
lies along high mountain ridges Oue to the rapid decrease
of moisture with height it will increase the thiekness of
the lower troposphere however the aseending motion in the
eb towers carries much heat upward to the 150-300 mb layer
- 20 shy - 21 shy
Most of this latent heat is used for heating up the upper
troposphere to a moist-adiabatic lapse-rate Since in SEshy
Tibet and Assam the average vertical lapse-rate is slightly
super-moist-adiabatie any ascending parcel of air in the
eondensation layer between 775 and 205 mb is about 16oC
warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in
the arid areas
The effect of the direct warming of air above southernmost
and western Tibet and north-eastern India has been qualitashy
tively stressed by the author (1950 1960 1968) Strong
evidenee has been contributed in a model eomputation by
MURAKAMI (1958) who started from the average 300 mb-flow
during May introduced a heat source above the whole
Tibetan plateau and obtained after a model time of 72
hours an antieyelonic eell in southeastern Tibet In a
more recent model of a two-dimensional meridional cross-
section along BOoE the role of the mountains and of the
hydrologie cycle has been investigated numerically by the
same author (1969) Using correlation analysis ASAKURA
(196B) recently demonstrated the strong persistence and
the large-seale influence of the Tibetan anticyelone at
the 500 mb-level
Careful analysis of all available aerological data shy
taking Into account the diurnal variation of upper air
temperatures as well as thermaI winds (FLOHN 196B) shy
revealed beyond any doubt the existenee of an elongated
heat zone in the upper troposphere (300-500 mb) above the
Himalayas and southern Tibet centered at 300 N and 850S
(Figure 5)
r e
N
~ I
2 I
~
abull
+ lt lJl middotri
t 0
Q) ~ III U Ul
Ul Cl U t 0
-rl ~
-1 Q) III gtlt
III ~ ~ Q)
c + 0
Q)Cl t nl l +
nl (l) (l)
gtltIII ~
-1 Q) +
~ t rtI Q)0
0 S If)
I 0 0 + 0 c M -rl
Ul t Ul Q) Q) t + 10( + U lt1l - p c
+ Il Ul Ul Q) + t 104
10( Ql Q ~ c 0
u If)
lJl r
- 22 shy
b) In the arid north-western part of the subcontinent (Sind
Rajasthan) the aridity is maintained even during the monsoon
season when moist air with an almost constant dewpoint near 0
25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days
when monsoon depressions travelling from the east or south
penetrate this area On all other days - ie during more
than 90 per cent of the season - the permanent and steady
monsoon flow is divergent turning from south-west above
Jodhpur and Gwalior to south-south-east above Jacobabad and
therefore subject to large-seale subsidence (FLOHN et al
196B) which reduces the vertical extension of the moist
layer to about 15-2 km and its eloud cover to shallow
stratocumuli (sc) This divergence can be understood if we
take into aceount the average position of the pressure trough
with its anticyelonic curvature from eastern Arabia to the
Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1
Similarly the SW-monsoon above the Arabian Sea is - at least
west of Long 65-68o
E - essentially rainless with a shallow
layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by
directional diffluence but by the increasing velocity along
the stream lines About 500-BOO kms off the Indian west coast
the wind speed decreases accompanied by a remarkable increase
of shower activity These facts strongly support our present
view of the dynamic effects of the wind vergences in contrast
to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)
cl The remarkable Foumlhn effect on the eastern side of the
West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a
frequency of 29-30 rain-days a month from June to September)
we observe only 20-40 km east of the crest not more than
- 23 shy
600-800 mm mostly produced by a few travelling synoptic
disturbances In some of the deepest valleys of the Himalaya
Mountains - even in the rainiest areas north of Assam - the
valley bottom is almost arid (as demonstrated by the vegeshy
tation) (SCHWEINFURTH 1956) since the local circulations
always control the pattern of wind components clouds and
rain (FLOHN 1970 DITTMANN 1970) This is also true in the
deep meridional gorges of Upper Burma - in spite of the high
cloudiness and moisture content of the air during late spring
and summer below the quasi-permanent anticyclonic cell in the
upper troposphere The slight anticyclonic curvature of the
lower westerlies crossing the Ghats (Figure 2) might also be
related to the permanent release of latent heat
d) The distribution of land- and sea-breezes controls a large
part of the convective activity The frequency and extend of
these diurnal circulations along all the coasts of the subshy
continent deserves much more attention together with their
effect on the weather During the summer monsoon these
diurnal circulations are to a large extend weakened or even
suppressed During the remaining seasons the quasi-geostrophic
flow near the surface is much weaker the diurnal radiation
cycle is more effective due to low cloudiness and the diurnal
circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)
middote) Careful analysis of the diurnal wind circulation at the
Himalayas led to the unexpected conclusion that the usual
reversal from the ascending day-time circulation to the
descending nighttime circulation is to a large extend
suppressed by the permanence of the above-mentioned heat
center in the upper troposphere during night (FLOHN 196B
1970) Thus in many valleys in the Himalayas (including the
meridional gorges) the weak nighttime mountain winds are
either superimposed with upstream winds or are replaced by calms
- 24 shy
D The retreat of the monsoon during September and October
(REITER amp HEUBERGER 1960) seems not to be merely a mirror
of the events during the onset in early June The large-scale
reversamiddotl of the wind field is somewhat more gradual and
relatively slow but low-Ievel cyclones still cross the
peninsula and may be steered in a northward direction causing
veritable cloudbursts and severe floods Due to the gradual
~~ifting of their tracks they often have the opportunity to
ltevelop over ocean areas into full-size warm-core hurricanes
notably over the Gulf of Bengal From October to December
tropical hurricanes and other cyclones of minor intensity
are fairly frequent here they can almost certainly be tracked
along their path from the east ie from the Gulf of Thailand
They are responsible for the late fall rains on the east coast
(and over the southcentral part) of the peninsula as weIl as
for the rainy season on all eastfacing coasts of south-east
Asia In contrast to spring they are in this season relatively
rare (but not absent) over the Arabian Sea Since the northshy
east flow itself is shallow and mostly stable at least north
of about latitude 12oN its effect on rainfall is small The
fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations
from year to year Since the cyclonic disturbances usually
travel towards west the shearing intensity reaches its maxishy
mum at their western side Together with the increase of
surface stress at the east-facing coasts this leads to a
marked rainfall maximum along these coasts
E In order to avoid further misunderstanding it should be
stressed that the monsoonal reversal of the surface winds is
by no means identical with begin and end of the rainy season
In fact the rainy season starts in the northeastern area of
the subcontinent about two months earlier than the reversal
of (upper) winds while in the heart of the peninsula the
- 25 shy
low-level wind shift precedes the onset of the monsoon rains
by about the same time
The early onset of rains in NE-India contributes - together
with the seasonal warming of the elevated Tibetan highlands shy
to the development of the high-tropospheric summer anticyclones
above the Himalayas This event produces the reversal of the
winds of the mlddle and upper troposphere which now triggers
the onset of the monsoon rains above the largest portion of
the subcontinent Such an interaction between thermodynamic
and dynamic processes is by no means uncommon in the largeshy
scale atmospheric circulation In this area it is not merely
a seasonal shift but an orographically fixed reversal of
temperature and pressure gradients which leads to such dramatic
(and far-reaching) consequences Before the existence of the
Tibetan highlands - ie before the late-Tertiary (and recent)
uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa
or Australia This is also evident from the recent model
computation of MURAKAMI (1969)
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
- 18 shy
S50 roh level Apparently lifting and consequently rainfall
reach a maximum in their southern section there is hardly
sufficient evidence of definite air-mass differences in their
area Generally speaking air-mass differences during the
monsoon season above the subcontinent are much smaller than
in middle latitudes and hardly detectable in the lower
troposphere However in the upper troposphere we observe a remarkable baroclinie structure where the equatorial air
above SON is about 7-10oC cooler than the subtropical air
above the heat eentre at 28-30oN This reversed temperature
gradient is eorrelated with an extremely persistent tropiealo
easterly jet (FLOHN 1964) centred near latitude IS N one or two km below the tropieal tropopause This easterly jet is
of remarkable persistence from day to day with only oecasional fluetuations of the wind velocity (pulses) whieh
have a duration of three to six days and varying between
about 40 and more than 100 knots in the centre According to
recent studies by RAMAN (1964) these pulses are eorrelated
with fluctuations of the rainfall above the subcontinent and thus with the varying release of latent heat which
intensifies the anticyelonie eells above the Himalayas
On the average the speed of the Tropical Easterly Jet inshy
creases (together with a northerly component of the winds)
east of 850 E and decreases (together with a weak southerly
eomponent) towards the west This causes (FLOHN 1964) a eross-isobaric meridional circulation reverting its sign
near 8SoE (Figure 4) Above SE Asia ascending motion with
strong cloudiness and rainfall prevails along the northern edge of the system (Lat 20-32oN) while subsiding motions
are frequently found along th~ southern edge near the equator Above SW-Asia and Africathe reverse is true (FLOHN
1964) large-scale subsidence tendency along the northern flane (including the area of the Intertropical Convergence
Zone at the surface) in contrast to lifting motions along
~
- 19 shy
the southern edge (the belt of highest rainfall along Lat
7-14oN)
Nevertheless climatic features near surfaee are by no means
unimportant in the large-scale meehanism of the summer-monsoon
rains in southern Asia We list only a few of them
a) The reversal of the temperature pressure and wind field
of the middle and upper troposphere is preceded by the gradual
warming of northern India the Tibetan highlands and the
adjaeent highlands Bere we have to distinguish two main
sources of internal and potential energy
1 The flux of sensible heat into the air from the elevated
heat souree of the arid (western and central) seetion of
the Tibetan highlands Here all terms of the heat balance
can be assumed to be nearly equal to those in the adjacent
arid lowlands of centra1 Asia Suffieient evidence for
this assumption has been presented by AIZENSHTAT (1966)
from many heat balance investigations in the arid high
mountains of the USSR at altitudes of 3100-4000 m The
average input of sensible heat into the atmosphere amounts
to 180-250 Lyday (1 Ly = 1 calcm2)
2 The release of latent heat from the enormous quantities
of rain falling on the superhumid mountain areas of Bengal
Assam and Upper Burma This has an estimated area-averaged value for the cloud layer of 900-1200 Lyd ie more
than the global radiation at the surfaee can yield Its
effect has been suggested generally (FLOHN 1953) as an
addition to the anticyclonic curvature of the upper westershy
lies along high mountain ridges Oue to the rapid decrease
of moisture with height it will increase the thiekness of
the lower troposphere however the aseending motion in the
eb towers carries much heat upward to the 150-300 mb layer
- 20 shy - 21 shy
Most of this latent heat is used for heating up the upper
troposphere to a moist-adiabatic lapse-rate Since in SEshy
Tibet and Assam the average vertical lapse-rate is slightly
super-moist-adiabatie any ascending parcel of air in the
eondensation layer between 775 and 205 mb is about 16oC
warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in
the arid areas
The effect of the direct warming of air above southernmost
and western Tibet and north-eastern India has been qualitashy
tively stressed by the author (1950 1960 1968) Strong
evidenee has been contributed in a model eomputation by
MURAKAMI (1958) who started from the average 300 mb-flow
during May introduced a heat source above the whole
Tibetan plateau and obtained after a model time of 72
hours an antieyelonic eell in southeastern Tibet In a
more recent model of a two-dimensional meridional cross-
section along BOoE the role of the mountains and of the
hydrologie cycle has been investigated numerically by the
same author (1969) Using correlation analysis ASAKURA
(196B) recently demonstrated the strong persistence and
the large-seale influence of the Tibetan anticyelone at
the 500 mb-level
Careful analysis of all available aerological data shy
taking Into account the diurnal variation of upper air
temperatures as well as thermaI winds (FLOHN 196B) shy
revealed beyond any doubt the existenee of an elongated
heat zone in the upper troposphere (300-500 mb) above the
Himalayas and southern Tibet centered at 300 N and 850S
(Figure 5)
r e
N
~ I
2 I
~
abull
+ lt lJl middotri
t 0
Q) ~ III U Ul
Ul Cl U t 0
-rl ~
-1 Q) III gtlt
III ~ ~ Q)
c + 0
Q)Cl t nl l +
nl (l) (l)
gtltIII ~
-1 Q) +
~ t rtI Q)0
0 S If)
I 0 0 + 0 c M -rl
Ul t Ul Q) Q) t + 10( + U lt1l - p c
+ Il Ul Ul Q) + t 104
10( Ql Q ~ c 0
u If)
lJl r
- 22 shy
b) In the arid north-western part of the subcontinent (Sind
Rajasthan) the aridity is maintained even during the monsoon
season when moist air with an almost constant dewpoint near 0
25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days
when monsoon depressions travelling from the east or south
penetrate this area On all other days - ie during more
than 90 per cent of the season - the permanent and steady
monsoon flow is divergent turning from south-west above
Jodhpur and Gwalior to south-south-east above Jacobabad and
therefore subject to large-seale subsidence (FLOHN et al
196B) which reduces the vertical extension of the moist
layer to about 15-2 km and its eloud cover to shallow
stratocumuli (sc) This divergence can be understood if we
take into aceount the average position of the pressure trough
with its anticyelonic curvature from eastern Arabia to the
Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1
Similarly the SW-monsoon above the Arabian Sea is - at least
west of Long 65-68o
E - essentially rainless with a shallow
layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by
directional diffluence but by the increasing velocity along
the stream lines About 500-BOO kms off the Indian west coast
the wind speed decreases accompanied by a remarkable increase
of shower activity These facts strongly support our present
view of the dynamic effects of the wind vergences in contrast
to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)
cl The remarkable Foumlhn effect on the eastern side of the
West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a
frequency of 29-30 rain-days a month from June to September)
we observe only 20-40 km east of the crest not more than
- 23 shy
600-800 mm mostly produced by a few travelling synoptic
disturbances In some of the deepest valleys of the Himalaya
Mountains - even in the rainiest areas north of Assam - the
valley bottom is almost arid (as demonstrated by the vegeshy
tation) (SCHWEINFURTH 1956) since the local circulations
always control the pattern of wind components clouds and
rain (FLOHN 1970 DITTMANN 1970) This is also true in the
deep meridional gorges of Upper Burma - in spite of the high
cloudiness and moisture content of the air during late spring
and summer below the quasi-permanent anticyclonic cell in the
upper troposphere The slight anticyclonic curvature of the
lower westerlies crossing the Ghats (Figure 2) might also be
related to the permanent release of latent heat
d) The distribution of land- and sea-breezes controls a large
part of the convective activity The frequency and extend of
these diurnal circulations along all the coasts of the subshy
continent deserves much more attention together with their
effect on the weather During the summer monsoon these
diurnal circulations are to a large extend weakened or even
suppressed During the remaining seasons the quasi-geostrophic
flow near the surface is much weaker the diurnal radiation
cycle is more effective due to low cloudiness and the diurnal
circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)
middote) Careful analysis of the diurnal wind circulation at the
Himalayas led to the unexpected conclusion that the usual
reversal from the ascending day-time circulation to the
descending nighttime circulation is to a large extend
suppressed by the permanence of the above-mentioned heat
center in the upper troposphere during night (FLOHN 196B
1970) Thus in many valleys in the Himalayas (including the
meridional gorges) the weak nighttime mountain winds are
either superimposed with upstream winds or are replaced by calms
- 24 shy
D The retreat of the monsoon during September and October
(REITER amp HEUBERGER 1960) seems not to be merely a mirror
of the events during the onset in early June The large-scale
reversamiddotl of the wind field is somewhat more gradual and
relatively slow but low-Ievel cyclones still cross the
peninsula and may be steered in a northward direction causing
veritable cloudbursts and severe floods Due to the gradual
~~ifting of their tracks they often have the opportunity to
ltevelop over ocean areas into full-size warm-core hurricanes
notably over the Gulf of Bengal From October to December
tropical hurricanes and other cyclones of minor intensity
are fairly frequent here they can almost certainly be tracked
along their path from the east ie from the Gulf of Thailand
They are responsible for the late fall rains on the east coast
(and over the southcentral part) of the peninsula as weIl as
for the rainy season on all eastfacing coasts of south-east
Asia In contrast to spring they are in this season relatively
rare (but not absent) over the Arabian Sea Since the northshy
east flow itself is shallow and mostly stable at least north
of about latitude 12oN its effect on rainfall is small The
fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations
from year to year Since the cyclonic disturbances usually
travel towards west the shearing intensity reaches its maxishy
mum at their western side Together with the increase of
surface stress at the east-facing coasts this leads to a
marked rainfall maximum along these coasts
E In order to avoid further misunderstanding it should be
stressed that the monsoonal reversal of the surface winds is
by no means identical with begin and end of the rainy season
In fact the rainy season starts in the northeastern area of
the subcontinent about two months earlier than the reversal
of (upper) winds while in the heart of the peninsula the
- 25 shy
low-level wind shift precedes the onset of the monsoon rains
by about the same time
The early onset of rains in NE-India contributes - together
with the seasonal warming of the elevated Tibetan highlands shy
to the development of the high-tropospheric summer anticyclones
above the Himalayas This event produces the reversal of the
winds of the mlddle and upper troposphere which now triggers
the onset of the monsoon rains above the largest portion of
the subcontinent Such an interaction between thermodynamic
and dynamic processes is by no means uncommon in the largeshy
scale atmospheric circulation In this area it is not merely
a seasonal shift but an orographically fixed reversal of
temperature and pressure gradients which leads to such dramatic
(and far-reaching) consequences Before the existence of the
Tibetan highlands - ie before the late-Tertiary (and recent)
uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa
or Australia This is also evident from the recent model
computation of MURAKAMI (1969)
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
- 20 shy - 21 shy
Most of this latent heat is used for heating up the upper
troposphere to a moist-adiabatic lapse-rate Since in SEshy
Tibet and Assam the average vertical lapse-rate is slightly
super-moist-adiabatie any ascending parcel of air in the
eondensation layer between 775 and 205 mb is about 16oC
warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in
the arid areas
The effect of the direct warming of air above southernmost
and western Tibet and north-eastern India has been qualitashy
tively stressed by the author (1950 1960 1968) Strong
evidenee has been contributed in a model eomputation by
MURAKAMI (1958) who started from the average 300 mb-flow
during May introduced a heat source above the whole
Tibetan plateau and obtained after a model time of 72
hours an antieyelonic eell in southeastern Tibet In a
more recent model of a two-dimensional meridional cross-
section along BOoE the role of the mountains and of the
hydrologie cycle has been investigated numerically by the
same author (1969) Using correlation analysis ASAKURA
(196B) recently demonstrated the strong persistence and
the large-seale influence of the Tibetan anticyelone at
the 500 mb-level
Careful analysis of all available aerological data shy
taking Into account the diurnal variation of upper air
temperatures as well as thermaI winds (FLOHN 196B) shy
revealed beyond any doubt the existenee of an elongated
heat zone in the upper troposphere (300-500 mb) above the
Himalayas and southern Tibet centered at 300 N and 850S
(Figure 5)
r e
N
~ I
2 I
~
abull
+ lt lJl middotri
t 0
Q) ~ III U Ul
Ul Cl U t 0
-rl ~
-1 Q) III gtlt
III ~ ~ Q)
c + 0
Q)Cl t nl l +
nl (l) (l)
gtltIII ~
-1 Q) +
~ t rtI Q)0
0 S If)
I 0 0 + 0 c M -rl
Ul t Ul Q) Q) t + 10( + U lt1l - p c
+ Il Ul Ul Q) + t 104
10( Ql Q ~ c 0
u If)
lJl r
- 22 shy
b) In the arid north-western part of the subcontinent (Sind
Rajasthan) the aridity is maintained even during the monsoon
season when moist air with an almost constant dewpoint near 0
25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days
when monsoon depressions travelling from the east or south
penetrate this area On all other days - ie during more
than 90 per cent of the season - the permanent and steady
monsoon flow is divergent turning from south-west above
Jodhpur and Gwalior to south-south-east above Jacobabad and
therefore subject to large-seale subsidence (FLOHN et al
196B) which reduces the vertical extension of the moist
layer to about 15-2 km and its eloud cover to shallow
stratocumuli (sc) This divergence can be understood if we
take into aceount the average position of the pressure trough
with its anticyelonic curvature from eastern Arabia to the
Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1
Similarly the SW-monsoon above the Arabian Sea is - at least
west of Long 65-68o
E - essentially rainless with a shallow
layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by
directional diffluence but by the increasing velocity along
the stream lines About 500-BOO kms off the Indian west coast
the wind speed decreases accompanied by a remarkable increase
of shower activity These facts strongly support our present
view of the dynamic effects of the wind vergences in contrast
to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)
cl The remarkable Foumlhn effect on the eastern side of the
West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a
frequency of 29-30 rain-days a month from June to September)
we observe only 20-40 km east of the crest not more than
- 23 shy
600-800 mm mostly produced by a few travelling synoptic
disturbances In some of the deepest valleys of the Himalaya
Mountains - even in the rainiest areas north of Assam - the
valley bottom is almost arid (as demonstrated by the vegeshy
tation) (SCHWEINFURTH 1956) since the local circulations
always control the pattern of wind components clouds and
rain (FLOHN 1970 DITTMANN 1970) This is also true in the
deep meridional gorges of Upper Burma - in spite of the high
cloudiness and moisture content of the air during late spring
and summer below the quasi-permanent anticyclonic cell in the
upper troposphere The slight anticyclonic curvature of the
lower westerlies crossing the Ghats (Figure 2) might also be
related to the permanent release of latent heat
d) The distribution of land- and sea-breezes controls a large
part of the convective activity The frequency and extend of
these diurnal circulations along all the coasts of the subshy
continent deserves much more attention together with their
effect on the weather During the summer monsoon these
diurnal circulations are to a large extend weakened or even
suppressed During the remaining seasons the quasi-geostrophic
flow near the surface is much weaker the diurnal radiation
cycle is more effective due to low cloudiness and the diurnal
circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)
middote) Careful analysis of the diurnal wind circulation at the
Himalayas led to the unexpected conclusion that the usual
reversal from the ascending day-time circulation to the
descending nighttime circulation is to a large extend
suppressed by the permanence of the above-mentioned heat
center in the upper troposphere during night (FLOHN 196B
1970) Thus in many valleys in the Himalayas (including the
meridional gorges) the weak nighttime mountain winds are
either superimposed with upstream winds or are replaced by calms
- 24 shy
D The retreat of the monsoon during September and October
(REITER amp HEUBERGER 1960) seems not to be merely a mirror
of the events during the onset in early June The large-scale
reversamiddotl of the wind field is somewhat more gradual and
relatively slow but low-Ievel cyclones still cross the
peninsula and may be steered in a northward direction causing
veritable cloudbursts and severe floods Due to the gradual
~~ifting of their tracks they often have the opportunity to
ltevelop over ocean areas into full-size warm-core hurricanes
notably over the Gulf of Bengal From October to December
tropical hurricanes and other cyclones of minor intensity
are fairly frequent here they can almost certainly be tracked
along their path from the east ie from the Gulf of Thailand
They are responsible for the late fall rains on the east coast
(and over the southcentral part) of the peninsula as weIl as
for the rainy season on all eastfacing coasts of south-east
Asia In contrast to spring they are in this season relatively
rare (but not absent) over the Arabian Sea Since the northshy
east flow itself is shallow and mostly stable at least north
of about latitude 12oN its effect on rainfall is small The
fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations
from year to year Since the cyclonic disturbances usually
travel towards west the shearing intensity reaches its maxishy
mum at their western side Together with the increase of
surface stress at the east-facing coasts this leads to a
marked rainfall maximum along these coasts
E In order to avoid further misunderstanding it should be
stressed that the monsoonal reversal of the surface winds is
by no means identical with begin and end of the rainy season
In fact the rainy season starts in the northeastern area of
the subcontinent about two months earlier than the reversal
of (upper) winds while in the heart of the peninsula the
- 25 shy
low-level wind shift precedes the onset of the monsoon rains
by about the same time
The early onset of rains in NE-India contributes - together
with the seasonal warming of the elevated Tibetan highlands shy
to the development of the high-tropospheric summer anticyclones
above the Himalayas This event produces the reversal of the
winds of the mlddle and upper troposphere which now triggers
the onset of the monsoon rains above the largest portion of
the subcontinent Such an interaction between thermodynamic
and dynamic processes is by no means uncommon in the largeshy
scale atmospheric circulation In this area it is not merely
a seasonal shift but an orographically fixed reversal of
temperature and pressure gradients which leads to such dramatic
(and far-reaching) consequences Before the existence of the
Tibetan highlands - ie before the late-Tertiary (and recent)
uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa
or Australia This is also evident from the recent model
computation of MURAKAMI (1969)
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
- 22 shy
b) In the arid north-western part of the subcontinent (Sind
Rajasthan) the aridity is maintained even during the monsoon
season when moist air with an almost constant dewpoint near 0
25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days
when monsoon depressions travelling from the east or south
penetrate this area On all other days - ie during more
than 90 per cent of the season - the permanent and steady
monsoon flow is divergent turning from south-west above
Jodhpur and Gwalior to south-south-east above Jacobabad and
therefore subject to large-seale subsidence (FLOHN et al
196B) which reduces the vertical extension of the moist
layer to about 15-2 km and its eloud cover to shallow
stratocumuli (sc) This divergence can be understood if we
take into aceount the average position of the pressure trough
with its anticyelonic curvature from eastern Arabia to the
Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1
Similarly the SW-monsoon above the Arabian Sea is - at least
west of Long 65-68o
E - essentially rainless with a shallow
layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by
directional diffluence but by the increasing velocity along
the stream lines About 500-BOO kms off the Indian west coast
the wind speed decreases accompanied by a remarkable increase
of shower activity These facts strongly support our present
view of the dynamic effects of the wind vergences in contrast
to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)
cl The remarkable Foumlhn effect on the eastern side of the
West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a
frequency of 29-30 rain-days a month from June to September)
we observe only 20-40 km east of the crest not more than
- 23 shy
600-800 mm mostly produced by a few travelling synoptic
disturbances In some of the deepest valleys of the Himalaya
Mountains - even in the rainiest areas north of Assam - the
valley bottom is almost arid (as demonstrated by the vegeshy
tation) (SCHWEINFURTH 1956) since the local circulations
always control the pattern of wind components clouds and
rain (FLOHN 1970 DITTMANN 1970) This is also true in the
deep meridional gorges of Upper Burma - in spite of the high
cloudiness and moisture content of the air during late spring
and summer below the quasi-permanent anticyclonic cell in the
upper troposphere The slight anticyclonic curvature of the
lower westerlies crossing the Ghats (Figure 2) might also be
related to the permanent release of latent heat
d) The distribution of land- and sea-breezes controls a large
part of the convective activity The frequency and extend of
these diurnal circulations along all the coasts of the subshy
continent deserves much more attention together with their
effect on the weather During the summer monsoon these
diurnal circulations are to a large extend weakened or even
suppressed During the remaining seasons the quasi-geostrophic
flow near the surface is much weaker the diurnal radiation
cycle is more effective due to low cloudiness and the diurnal
circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)
middote) Careful analysis of the diurnal wind circulation at the
Himalayas led to the unexpected conclusion that the usual
reversal from the ascending day-time circulation to the
descending nighttime circulation is to a large extend
suppressed by the permanence of the above-mentioned heat
center in the upper troposphere during night (FLOHN 196B
1970) Thus in many valleys in the Himalayas (including the
meridional gorges) the weak nighttime mountain winds are
either superimposed with upstream winds or are replaced by calms
- 24 shy
D The retreat of the monsoon during September and October
(REITER amp HEUBERGER 1960) seems not to be merely a mirror
of the events during the onset in early June The large-scale
reversamiddotl of the wind field is somewhat more gradual and
relatively slow but low-Ievel cyclones still cross the
peninsula and may be steered in a northward direction causing
veritable cloudbursts and severe floods Due to the gradual
~~ifting of their tracks they often have the opportunity to
ltevelop over ocean areas into full-size warm-core hurricanes
notably over the Gulf of Bengal From October to December
tropical hurricanes and other cyclones of minor intensity
are fairly frequent here they can almost certainly be tracked
along their path from the east ie from the Gulf of Thailand
They are responsible for the late fall rains on the east coast
(and over the southcentral part) of the peninsula as weIl as
for the rainy season on all eastfacing coasts of south-east
Asia In contrast to spring they are in this season relatively
rare (but not absent) over the Arabian Sea Since the northshy
east flow itself is shallow and mostly stable at least north
of about latitude 12oN its effect on rainfall is small The
fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations
from year to year Since the cyclonic disturbances usually
travel towards west the shearing intensity reaches its maxishy
mum at their western side Together with the increase of
surface stress at the east-facing coasts this leads to a
marked rainfall maximum along these coasts
E In order to avoid further misunderstanding it should be
stressed that the monsoonal reversal of the surface winds is
by no means identical with begin and end of the rainy season
In fact the rainy season starts in the northeastern area of
the subcontinent about two months earlier than the reversal
of (upper) winds while in the heart of the peninsula the
- 25 shy
low-level wind shift precedes the onset of the monsoon rains
by about the same time
The early onset of rains in NE-India contributes - together
with the seasonal warming of the elevated Tibetan highlands shy
to the development of the high-tropospheric summer anticyclones
above the Himalayas This event produces the reversal of the
winds of the mlddle and upper troposphere which now triggers
the onset of the monsoon rains above the largest portion of
the subcontinent Such an interaction between thermodynamic
and dynamic processes is by no means uncommon in the largeshy
scale atmospheric circulation In this area it is not merely
a seasonal shift but an orographically fixed reversal of
temperature and pressure gradients which leads to such dramatic
(and far-reaching) consequences Before the existence of the
Tibetan highlands - ie before the late-Tertiary (and recent)
uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa
or Australia This is also evident from the recent model
computation of MURAKAMI (1969)
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
- 24 shy
D The retreat of the monsoon during September and October
(REITER amp HEUBERGER 1960) seems not to be merely a mirror
of the events during the onset in early June The large-scale
reversamiddotl of the wind field is somewhat more gradual and
relatively slow but low-Ievel cyclones still cross the
peninsula and may be steered in a northward direction causing
veritable cloudbursts and severe floods Due to the gradual
~~ifting of their tracks they often have the opportunity to
ltevelop over ocean areas into full-size warm-core hurricanes
notably over the Gulf of Bengal From October to December
tropical hurricanes and other cyclones of minor intensity
are fairly frequent here they can almost certainly be tracked
along their path from the east ie from the Gulf of Thailand
They are responsible for the late fall rains on the east coast
(and over the southcentral part) of the peninsula as weIl as
for the rainy season on all eastfacing coasts of south-east
Asia In contrast to spring they are in this season relatively
rare (but not absent) over the Arabian Sea Since the northshy
east flow itself is shallow and mostly stable at least north
of about latitude 12oN its effect on rainfall is small The
fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations
from year to year Since the cyclonic disturbances usually
travel towards west the shearing intensity reaches its maxishy
mum at their western side Together with the increase of
surface stress at the east-facing coasts this leads to a
marked rainfall maximum along these coasts
E In order to avoid further misunderstanding it should be
stressed that the monsoonal reversal of the surface winds is
by no means identical with begin and end of the rainy season
In fact the rainy season starts in the northeastern area of
the subcontinent about two months earlier than the reversal
of (upper) winds while in the heart of the peninsula the
- 25 shy
low-level wind shift precedes the onset of the monsoon rains
by about the same time
The early onset of rains in NE-India contributes - together
with the seasonal warming of the elevated Tibetan highlands shy
to the development of the high-tropospheric summer anticyclones
above the Himalayas This event produces the reversal of the
winds of the mlddle and upper troposphere which now triggers
the onset of the monsoon rains above the largest portion of
the subcontinent Such an interaction between thermodynamic
and dynamic processes is by no means uncommon in the largeshy
scale atmospheric circulation In this area it is not merely
a seasonal shift but an orographically fixed reversal of
temperature and pressure gradients which leads to such dramatic
(and far-reaching) consequences Before the existence of the
Tibetan highlands - ie before the late-Tertiary (and recent)
uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa
or Australia This is also evident from the recent model
computation of MURAKAMI (1969)
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
- 26 shy
References
AIZENSHTAT BA 1966 Investigations on the heat budget of
Central Asia
In MI BUDYKO Actual problems of climatology
(russ) Leningrad 94-129
ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of
~ow level circulation over India
Proc WMO SymposTropMeteor Rotorua NZ 5-13
Nov 1963 New Zealand Meteor Service Wellington 89-95
ASAKURA T 1968 Dynamic climatology of atmospheric circushy
lation over East Asia centered in Japan
Papers in Meteorology and Geoohysics 19 1-68
DITTMANN E 1970 Statistische Untersuchungen zur Struktur
der Niederschlaumlge in Nepal
In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60
FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche
Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S
FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die
Gebirge als Waumlrmequellen
Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy
gie Sero A 5 265-279
FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy
tarischen Zirkulation der Atmosphaumlre
Berichte des Deutschen Wetterdienstes Nr 22 134-139
FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien
Erdkunde 12 294-308
- 27 shy
FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien
Aktuelle Schneegrenze und Sommerklima
Akademie der Wissenschaften und der Literatur
Abhandlungen der mathematisch-naturwissenschaftlichen
Klasse Nr 14 Mainz 1409-1431
FLOHN H 1960 Monsoon winds and general circulation
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1960 Recent investigations on the mechanism of the
summer monsoon of southern and eastern Asia
In Monsoons of the world Sympos Meteor Office
New Delhi 19-21 Febr 1958 New Delhi 65-74
FLOHN H 1964 Investigations on the tropical easterly jet
Bonner Meteorologische Abhandlungen Heft 4 83 pp
Flohn H 1965 Comments on a synoptic climatology of southern
Asia
WMO Techn Note No 69 245-252
FLOHN H 1968 Contributions to a meteorology of the Tibetan
Highlands
Colorado State Univ Fort Collins Atmosph Sei
Papers No 130 120 pp
FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs
und der benachbarten Hochgebirge
Erdkunde 23 205-215
FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya
In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47
FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics
or subsidence processes in the Arabian Sea summer
monsoon
J Atmosph Sci 25 527-529
KRISHNAMURTI TS 1961 1he subtropical jet stream of winter
J Meteor 18 172-191
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
- 28 shy
MURAKAMI T 1958 The sudden change of upper westerlies
near the Tibetan plateau at the beginning of summer
season
J of the Meteorological Society of Japan 36 239-247
MURAKAMI T and Coll 1969 unpubl report Univ Hawaii
Dept of Geosciences
RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy
lation and rainfall over Indo-Pakistan
J Meteor 9 176-179
RAMAGE CS 1952 Relationship of general circulation to
normal weather over southern Asia and the western
Pacific during the cool season
J Meteor 9 403-408
RAMAN CR and Y RAMANATHAN 1964 Interaction between lower
and upper tropical tropospheres
Nature 204 Nr 4953 31-35
RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~
Tellus 8 26-60
REITER ER and H HEUBERGER 1960 A synoptic exampe of the
retreat of the Indian summer monsoon
Geogr Ann 42 Nr 1 Stockholm 17-35
RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp
SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im
Himalaya
Erdkunde 10 297-302
SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the
climatology of South Asia
Wiesbaden Steiner 16 pp 15 maps
- K216_8und9
- K216_ohne_8und9
-
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