progress of the seasonal evolution of east asian summer ...asian summer monsoon, the delay of the...
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192 ACTA METEOROLOGICA SINICA VOL.19
Progress of the Seasonal Evolution of East Asian Summer
Circulation during July-August and Its Linkages with the
Subseasonal Processes over the Western North Pacific∗
LIAO Qinghai1(�����
), TAO Shiyan1( ���� ), and LIN Yonghui2( ���� )1Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029
2Chinese Academy of Meteorological Sciences, Beijing 100081
(Received July 8, 2004; revised November 10, 2004)
ABSTRACT
Based on the NCAR/NCEP monthly and pentad reanalysis dataset of 1961-2003, the progress of seasonalevolution of the summer atmospheric circulation in the East Asia in July to August, including the advancedand delayed cases, and their relationships with the subseasonal processes over the western North Pacific areanalyzed and compared with that of climatology. The results show that the progress of seasonal cycle isadvanced about a month ahead of the climatological time when the convection during 20-29 July is active inthe region of the subtropical West Pacific (15◦-25◦N, 150◦-165◦E), while it is delayed about one month whenweaker convections appear in the same region. Instead, the relative active convection for the latter occursin Pentad 46 (14-18 August). It is proved that the convective activities in the early July in the equatorialcentral and east Pacific, and then the convective anomalies in the subtropical western North Pacific canexcite the formation of the acceleration and delay of the seasonal circulation evolution in the East Asia inthe late summer. The preceding subseasonal processes over the western North Pacific, including the time-laginteractions among the active convection in the late June and early July, the Northwest Pacific anticyclone,the underlying sea surface temperature and low-level winds anomalies, and their relationships with theanomalous seasonal evolution of the summer atmospheric circulation in the East Asia in late July are alsoinvestigated. However, further study, especially the numerical experiments, is needed on the mechanism ofthe anomaly summer seasonal cycle in the East Asia and the Northwest Pacific.
Key words: seasonal cycle, convection, subseasonal anomalies
1. Introduction
The Asian summer monsoons are basically a re-
sponse of the atmosphere to the differential heating
between the land of the Asian Continent and the ad-
jacent oceans (Chen et al., 1992; Ding, 1994; Ye et al.,
1996). The atmospheric response, however, may be
quite complicated due to the complex topography in
the Asia, land-sea interaction, convection in the West
Pacific, including their interactions, especially in East
Asia. While the evolution of the East Asian summer
monsoon is largely phase-locked with the seasonal cy-
cle, a failure of the climatological seasonal cycle, which
leads to the hydrological disasters such as floods and
severe droughts, is not unusual. Tao and Xu (1962)
pointed out in the early 1960s that the seasonal vari-
ation of the atmospheric circulation in the flood years
in Jianghuai River Region would be delayed for one
month or so. Park and Schubert (1997) also indicated
that the seasonal cycle is advanced one month or so in
the year of severe droughts in the east of China, Japan
and the south of Korea in 1994. Although the above
results proved such facts that the unusual seasonal cy-
cle of the Asian summer monsoon is very important
for the hydrological disasters in the East Asian coun-
tries, the forcing mechanisms responsible for such an
extreme failure of the monsoon are yet to be fully un-
derstood. The further studies of this question have
significant practical meaning.
In addition, the air-sea interaction and convective
activity (Nitta et al., 1986; Nitta, 1987, 1990; Huang
and Li, 1987; Huang and Sun, 1992) also contribute to
the complexity of the seasonal cycle in the East Asian
monsoon region and the Northwest Pacific (Ueda et
∗This research is partly supported by the National Natural Science Foundation of China for Outstanding Young Scientists underGrant No. 40125014 and funded by the National Natural Science Foundation of China under Grant No. 40105010.
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NO.2 LIAO Qinghai, TAO Shiyan and LIN Yonghui 193
al., 1995; Ueda and Yasunali, 1996; Ose, 1998; Lu,
2000, 2001a, b). Based on TBB data from the Meteo-
rological Research Institute of Japan, He et al. (1996)
studied the features of seasonal transition of Asian-
Australian monsoons and Asian summer monsoon es-
tablishment, indicating that the transition begins as
early as April, followed by abrupt changes in May-
June; the Asian summer monsoon is fully established
in June. The winter convective center in Sumatra
moves steadily northwestward across the “land bridge”
of the maritime continent and the Indo-China Penin-
sula as time goes from winter to summer, thus giving
rise to the change in large scale circulations that is re-
sponsible for the summer monsoon establishment over
the Southeast Asia and India; the South China Sea
to the western Pacific summer monsoon onset bears
a close relation to the active convection in the Indo-
China Peninsula and the steady eastward retreat of the
western Pacific subtropical high. The abrupt north-
ward shift of active convective region from 10◦N to
25◦N around 150◦E during 25-29 July seen in the sea-
sonal cycle of the summer monsoon over the western
North Pacific (convection jump) and other associated
phenomena preceding this convection jump are inves-
tigated in detail by Ueda et al. (1995, 1996) for the pe-
riod of 1980-1994. They noted that the tongue-shaped
warm sea surface temperatures (SST) areas (warmer
than 29 ◦C) are observed in early July around 20◦N,
150◦-160◦E, preceding the typical convection jump.
This warming of SST is closely related to the appear-
ance of a weak wind region (weakening of easterlies)
around 25◦N, 140◦-160◦E in late June. These weak
easterlies are likely to be associated with the propa-
gation of Rossby wave induced by the occurrence of
active convection near the Philippine Islands in the
middle to late June. Ose (1998) investigated with
a set of idealized numerical experiments on whether
the seasonal change of the prescribed diabatic heating
reproduces the seasonal change of the Asian summer
monsoon circulation from July to August. Seasonal
change from mid-summer (July) to late summer (Au-
gust) is characterized by enhanced convective activity
in the extended area of the subtropical western Pacific.
The change of the heat source over the western Pacific
solely explains the major characteristics of the clima-
tological seasonal change from July to August not only
over the Pacific but also over the Indian Ocean. The
expansion of the Tibetan High at upper-level and the
Pacific High at low-level over Japan is also simulated
only by the seasonal change of the western Pacific heat
sources. On the interannual scale, Lu (2000, 2001a,
b) studied the linkages between the atmospheric cir-
culations and SST related to the convection over the
Western Pacific Warm Pool (WPWP, 10◦-20◦N, 110◦-
160◦E). The intensity of outgoing longwave radiation
(OLR) over WPWP is well correlated with the pre-
ceding (the previous winter and spring) SSTs over the
West Pacific region (10◦-20◦N, 130◦-170◦E) and the
equatorial central and East Pacific (5◦S-5◦N, 180◦-
90◦W), but not on the same term, and vice versa for
the SST over the west of the Philippine Islands.
The datasets used in this study are:
(1) the reanalysis monthly and pentad (5-day
averaged, 73 pentads per year. Each year has
365 days with no leap day) datasets of 1961-2003
from NCEP/NCAR (National Centers for Environ-
ment Prediction/National Center for Atmospheric Re-
search) (Kalnay et al., 1996);
(2) OLR data of 1979-2003 from the NOAA;
(3) weekly SST, 1982-2003 (Reynolds and Smith,
1995; Smith and Reynolds, 1998).
An index of the acceleration and delay of the sea-
sonal evolution in the East Asia is defined, and the
circulation characteristics in the late July over the
East Asia and the Northwest Pacific are further in-
vestigated on the subseasonal and interannual scales.
The indexes used here and the corresponding circula-
tion characteristics are described in Section 2. The
variability of both OLR and weekly SST and their
linkages with the interannual variability of the index
are documented by the composite and lag-correlation
analyses in Section 3. Section 4 examines the possible
associations between the SST, convections and the cir-
culations in the low-level atmosphere. The conclusions
are summarized in Section 5.
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194 ACTA METEOROLOGICA SINICA VOL.19
2. The atmospheric circulations associated
with the progress of the seasonal cycle of
the summer circulations in the East Asia
To investigate the acceleration and delay in the
climatological seasonal evolution in late summer, the
climatological features of atmospheric circulation pat-
terns of the eddy height at 200 hPa are analyzed. The
results show that in July, the South Asian high center
in the southeast of the Tibetan Plateau in June con-
tinues to move northwestward, and a new west high
center develops over the Iran-Afghanistan region. The
whole South Asian high shrinks and becomes stronger
in July. The trough in the East Asia weakens and fur-
ther westward migrates, then a low center develops in
the south of the Baikal Lake. The mid-ocean upper
level trough also becomes much stronger and enlarged
in July. In August, the circulation patterns continue
to change. The west center of the high in July weak-
ens, and the corresponding monsoon rainfall (Meiyu)
ends in the Middle and Lower Reaches of the Yangtze
River and Huaihe River. The most pronounced change
from July to August, however, is over the Northwest
Pacific region, centered over the Japan Islands, where
the upper-level trough is replaced by a ridge (a local
high center) in August. The low center in the south
of the Baikal Lake continues to develop into a long
low band covering the regions across the Atlantic, the
Mediterranean Sea, and the south of Lake Baikal (Park
and Schubert, 1997; Tao and Chen, 1987). Figure 1
can rather illustrate the seasonal evolution of the mid-
dle latitude eddy height or stationary waves. While
the seasonal evolution of the upper-level anticyclone
around the Tibetan Plateau (also called the monsoon
high) is a dominant feature in the eastern Hemisphere
during the summer, the cross sections at 40◦N in Fig.1
emphasize the evolution of stationary waves over the
East Asian sector. This is at the northern periphery
of the Tibetan anticyclone. The most pronounced sea-
sonal evolution in August is the rapid development of
Fig.1. Longitude-pressure sections of the eddy height climatology at 40◦N for (a) June, (b) July, and (c)August. Contour interval is 20 gpm.
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NO.2 LIAO Qinghai, TAO Shiyan and LIN Yonghui 195
the anticyclone near 150◦E, which is somewhat sepa-
rated from the South Asian anticyclone. The devel-
opment of this upper-level high centered around the
Japan Islands and Sea substantially changes the mid-
dle latitude circulation in the East Asia including the
vertical structure, which shows a strong dipole over
the West Pacific around 150◦E in August.
In order to describe the interannual variability of
the fast or slow seasonal evolution in late summer in
East Asia, a simple index is defined as the averaged
200 hPa eddy height in July over the key region ( 30◦-
50◦N, 120◦-160◦E). If the circulation patterns of July
in a year are similar to those of the climatology in Au-
gust, then the summertime seasonal cycle in this year
is accelerated. On the other hand, if the circulation
patterns of July in a year are similar to those of the
climatology in June, the summertime seasonal cycle
in this year is considered to be delayed. In the East
Asian summer monsoon, the delay of the seasonal cy-
cle means that the summer monsoon and rainfall per-
sist in the Huaihe River and the Middle and Lower
Reaches of the Yangtze River. The accelerated sea-
sonal cycle means the earlier ending of the Meiyu, and
also the earlier ending of the East Asian summer mon-
soon in the same regions.
Figure 2a indicates the normalized time series of
200 hPa eddy height in July averaged over the key re-
gion during 1961-2003. Years with values greater than
1.0 are selected as the positive anomaly years: 1961,
1962, 1963, 1967, 1972, 1973, 1977, 1978, and 1994,
while years with values less than -1.0 are selected
as the negative anomaly years: 1979, 1982, 1983, 1986,
Fig.2. (a) Time series of the normalized 200 hPa eddy height in July averaged over the key region (30◦-50◦N, 120◦-160◦E) during 1961-2003, and (b) the correlation coefficients between the above series and theeddy height differences which are the eddy height in July minus the climatological eddy height in August at40◦N. Light and heavy shadings indicate that their correlation coefficients exceed 95% and 99% confidencelevels, respectively.
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196 ACTA METEOROLOGICA SINICA VOL.19
Fig.3. The composite charts of the monthly eddy geopotential height for the positive (a-c) and negativecases (d-f) at 40◦N in June, July, and August (JJA).
1991, 1992, 1993, and 2003. Figure 2b shows the
correlation coefficients between the above series and
the eddy height differences which are the eddy height
in July minus the climatological eddy height in Au-
gust at 40◦N. Light and heavy shadings indicate that
their correlation coefficients exceed 95% and 99% con-
fidence levels, respectively. The positive eddy height
anomalies in the key region in July are associated with
the positive eddy height anomalies in the 120◦-160◦E,
especially in the upper troposphere, while negative
height anomalies in the middle-low levels of the tropo-
sphere in the area of 60◦-120◦E, and vice versa. The
above results are confirmed by the composite anal-
ysis in the next context. Figure 3 is the composite
charts of the monthly geopotential height for the posi-
tive and negative cases at 40◦N in June, July, and Au-
gust (JJA). A strong anticyclone develops rapidly for
the positive anomaly case in July, which is the same as
the climatology in August. The patterns in the nega-
tive case in July are similar to the climatology in June.
No anticyclone develops at 150◦E, and the mid-ocean
trough is weaker than that of the climatology. The
seasonal variational differences between the positive
and negative cases in the East Asia show a large zonal
asymmetric component, which means there exists pro-
nounced zonal asymmetric forcing. We will discuss it
in the later context.
3. Variations of OLR and SST in the equato-
rial central Pacific and subtropical west-
ern North Pacific
Ueda et al. (1995, 1996) stated that the sud-
den onset of the summer monsoon over the subtrop-
ical western North Pacific in late July is associated
with an abrupt northward shift of enhanced convec-
tion around 20◦N, 150◦E−“convective jump”. The
composite OLRA for the positive minus negative cases
is shown in Fig.4a. There are centers over the equa-
torial central Pacific, the subtropical Northwest
Pacific east of the Philippine Islands, especially east of
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NO.2 LIAO Qinghai, TAO Shiyan and LIN Yonghui 197
150◦E, and the southeast of the exit of the East Asian
Jet Streams. The time series of OLR over the east cen-
ter around 150◦E in the subtropical Northwest Pacific
are made for the positive cases, negative cases and the
climatology through Pentad 36 to 50 (Fig.4b). The
seasonal change of the climatological case reveals a re-
markable minimum in Pentad 42 (25-29 July), which
indicates an enhanced convection in this pentad. An
enhanced convection occurs in Pentad 41 (20-24 July)
for the positive cases, while in Pentad 46 (14-18 Au-
gust) a minimum of OLRA (OLR anomalies) appears
for the negative cases. A gap is pronounced for the
two kinds of cases. No big convection differences exist
between the positive cases and the climatology on the
subseasonal time scale.
The enhancement of convection in Pentads 41-
42 may be possible to associate with preceding SST
anomalies in the West Pacific. A longitude-time sec-
tion of linear correlation coefficients between the OLR
in Pentads 42-43 averaged over the same area of Fig.4b
and the weekly SST in 15◦-25◦N is shown in Fig.5. The
significant negative correlations around early June and
early to mid July clearly demonstrate that positive
SSTA is correlated to the enhanced convection (neg-
ative OLRA) in the early June over the subtropical
central and East Pacific and also an enhanced convec-
tion in the early and mid July (Week numbers 5-6)
over the region of 165◦E-170◦W. On the other hand,
a positive correlation in late July (Week numbers 8-9)
means the negative SSTA over the subtropical
Fig.4. (a) The composite OLRA for the positive minus negative cases. (b) The time series of OLR over thekey region (15◦-25◦N, 150◦-165◦E) in the subtropical Northwest Pacific for the positive cases (solid line withopen square), negative cases (solid line with dark square) and the climatology (solid line with dark circle)through Pentads 30 to 50. Pentad 30 corresponds to 26-30 May, while Pentad 50 to 3-7 August.
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198 ACTA METEOROLOGICA SINICA VOL.19
Fig.5. A longitude-time section of linear correlation coefficients between the OLR in Pentads 42-43 averagedover the key region and the weekly SST in 15◦-25◦N in the northern summer. Number 1 in the ordinaterepresents the first week of June, and 12 represents the last week of August. The shading indicates correlationcoefficients exceeding 95% confidence level.
Northwest Pacific matches with the active convection
in the region of 15◦-25◦N, 145◦-160◦E. That is SST
decreases after the enhanced convection. The decrease
of radiation for the existence of the convective clouds,
strong evaporation cooling and upwelling of cold sea
water due to the increase of wind speeds lead to the
above SST decreases.
Figure 6 indicates the time-lag correlations be-
tween the 200 hPa eddy height averaged over the key
region at Pentads 42-43 and the OLR averaged in Re-
gion 1 (5◦S-5◦N, 170◦-190◦E), Region 2 (15◦-25◦N,
150◦-165◦E), Region 3 (10◦-20◦N, 100◦-120◦E), and
Fig.6. The time-lag correlations between the 200 hPa eddy height averaged over the key region at Pentads42-43 and the OLR averaged in Region 1 (solid line with dark circle, 5◦S-5◦N, 170◦-190◦E), Region 2 (solidline with dark square, 15◦-25◦N, 150◦-165◦E), Region 3 (dashed line with no marker, 10◦-20◦N, 100◦-120◦E),and Region 4 (solid line with open circle, 7.5◦-17.5◦N, 115◦-125◦E). The time-lag correlations between OLRin Region 2 and Region 1 (solid line with open square) are also presented. Pentad 31 corresponds to 31 Mayto 4 June; Pentad 49 corresponds to 29 August to 2 September.
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NO.2 LIAO Qinghai, TAO Shiyan and LIN Yonghui 199
Region 4 (7.5◦-17.5◦N, 110◦-125◦E), which lie respec-
tively in the equatorial central Pacific, the subtropical
western North Pacific, the west of the Philippine Is-
lands, and the region around the Philippine Islands.
Significant (above 95% significance level) positive cor-
relation coefficients exist in Pentads 36-42 in Region 1.
Instead, negative correlation coefficients exist in Pen-
tads 38-43 in Region 2. Negative correlation between
OLR in Regions 2 and 1 demonstrates the existence of
a convective wave-like pattern during the early July to
late July and early August. The above results prove
that the convective activities in the early July in the
equatorial central and East Pacific, and then the con-
vective anomalies in the subtropical western North Pa-
cific can excite the formation of the acceleration and
delay of the seasonal circulation evolution in East Asia
in the late summer.
Previous researches (Nitta et al., 1986; Nitta,
1987, 1990; Kurihara and Tsuyuki, 1987; Huang and
Sun, 1992) also pointed the importance of the convec-
tive activity around the Philippine Islands. However,
no significant correlations exist between the height in-
dex (Fig.2) and the OLR variation in the west of the
Philippine Islands or around the Philippine Islands
during the late June and mid July until Pentads 43-
44 and Pentad 46. The above results indicate that
the convective activities in the west of and around the
Philippine Islands in the early summer contribute lit-
tle to the formation of the acceleration and delay of
the seasonal circulation evolution in East Asia in the
late summer.
4. Low-level circulation patterns associated
with OLR and SST anomalies
In the previous section, we have shown the strong
relationship between the height change at 200 hPa over
the key region and the convections, and weekly SST
in the equatorial central Pacific and subtropical west-
ern North Pacific. The variation of SSTA in the two
regions occurs under the influence of the Subtropical
High in the Pacific. Figure 7 depicts the linear regres-
sion fields of 850 hPa geopotential height in Pentad
36, wind differences and 850 hPa height change be-
tween Pentads 36-37 and Pentads 34-35 (Fig.7a), and
850 hPa geopotential height in Pentad 42, wind differ-
ences and 850 hPa height change between Pentads 42-
43 and Pentads 40-41 (Fig.7b) against the normalized
time series of the OLR over Region 2 in the context
based on the 1-σ standard. In the late June and early
July, the Subtropical High extends westward. Posi-
tive 850 hPa height change appears in the end of west
part of the Subtropical High, while negative height
change around 160◦E in the subtropics. The low-level
winds in the above two regions match with the increase
of SST in the west part of the Subtropical High and
with the decrease of SST around 160◦E in the subtrop-
ics. The convection enhanced around the Philippine
Islands and the Rossby wave propagation contribute to
the strengthening of the west part of the Subtropical
High. About 20 days later, due to continuous impacts
from the negative height change, the decrease of low
level winds and the Rossby wave excited by the
Fig.7. Linear regression fields of 850 hPa geopotential height at Pentad 36, wind differences and 850 hPaheight change between Pentads 36-37 and Pentads 34-35 (a), and 850 hPa geopotential height in Pentad42, wind differences and 850 hPa height change between Pentads 42-43 and Pentads 40-41 (b) (light shadesindicate negative height anomalies and heavy shades positive anomalies) against the normalized time seriesof the OLR over Region 2 in the context based on the 1-σ standard.
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200 ACTA METEOROLOGICA SINICA VOL.19
convection in the equatorial central Pacific, the SST
in the west of the subtropics continues to increase.
Then the convection in the subtropics around 160◦E
becomes more active, and the last response is the pos-
itive height change in the upper troposphere (Gill,
1980).
5. Concluding remarks
Through the analyses of convection and weekly
sea surface temperature in the subtropical western and
central Pacific, the possible mechanism for the anoma-
lies of the seasonal evolution of the atmospheric circu-
lations in East Asia in the late July has been discussed.
The roles of the convection around 160◦E in the west of
subtropics and the convection in the equatorial central
Pacific, SST anomalies and their subseasonal evolution
in the subtropics, their interactions with the low-level
subtropical high, and the continuing impacts from the
Rossby wave exited by the convective anomalies in
the above two regions are emphasized. The Rossby
wave patterns found by Nitta (1987) and Huang and
Sun (1992) are important only in early and middle
summer. The climatological change of the convection
around 20◦N, 160◦E is similar to the typical convec-
tion jump in Ueda (1995, 1996), but the convection
activity for the accelerated and delayed seasonal cycle
of the late summer circulation evolution has quite dif-
ferent meanings. Time-lag correlation and regression
analyses indicate that in the positive cases, the western
part of the Subtropical High around 160◦E is weakened
and the west tip of the high system becomes stronger
in late June, the convection in the equatorial central
Pacific continuous to impact the low-level subtropical
circulation in the first region, both of which lead to
the increase of SST in the western North Pacific, then
the convection is enhanced. The enhanced convective
activities trigger the formation of the local high over
the Japan Islands in the upper troposphere. The local
high around the Japan Islands pushes the Meiyu or
Baiu front northward, then ends the monsoon rainfall
in the regions of the Yangtze and Huaihe Rivers. Fur-
ther investigations will be needed on the formation of
the local high over and around Japan (Kawamura et
al., 1998), the impacts of El Niño on the convection,
especially over the equatorial central Pacific and the
MJO (Madden-Julian Oscillation) propagating from
the Indian Ocean and equatorial West Pacific.
Acknowledgments. The authors will be grate-
ful to the two anonymous reviewers for many valuable
comments and suggestions, which are helpful to the
improvement of this paper.
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