variability of the indian summer monsoon in relation to oceanic heat budget over the indian seas

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ELSEVIER Dynamics of Atmospheres and Oceans 21 (1994) 1-22

Variability of the Indian summer monsoon in relation to oceanic heat budget over the Indian seas

U . C . M o h a n t y *, K.J . R a m e s h , N. M o h a n K u m a r , K .V.J . P o t t y

Centre for Atmospheric Sciences, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi-llO 016, India

Received 24 February 1992; revised 22 July 1993; accepted 1 September 1993

Abstract

The influence of the surface marine meteorological parameters and air-sea fluxes of heat and moisture over the Indian seas (Bay of Bengal, Arabian Sea and equatorial Indian Ocean) on the interannual variability of the Indian summer monsoon has been studied in detail. The data base for this study, which consists of mean monthly marine meteorological fields for a period of 30 years (1950-1979), forms a part of the Comprehensive Ocean Atmospheric Data Set (COADS) analysed on a regular 1" lati tude/longitude resolution.

Using the surface marine meteorological fields, different components of the oceanic heat budget were computed. The basic meteorological fields, as well as the components of the oceanic heat budget, over the Indian seas were examined for the contrasting years of the summer monsoon (large excess rainfall:flood monsoon and large deficient rainfall:drought monsoon). To ascertain which regions were statistically significant in terms of heat budget over the Indian seas, the difference fields between two extreme categories of monsoon were subjected to Student's t-test.

The study revealed that in the month of May preceding a flood monsoon, there were stronger surface winds and more abundant cloud cover over most parts of the Arabian Sea compared with a corresponding period prior to a drought season over India. Thus, as a result of enhanced evaporation and reduction of incoming solar radiation during the pre-monsoon month of May, the difference fields of the oceanic heat budget between the extreme years (flood/drought) illustrate a statistically significant zone of heat loss over the entire equatorial Arabian Sea. The analysis during the month of May provides a useful qualitative indication of the subsequent mean monsoon activity over the Indian sub-continent. With the commencement of the monsoon (June-August) and its northeastward progress over the Arabian Sea, all the difference fields of oceanic heat budget maintain the

* Corresponding author. Present address: National Centre for Medium Range Weather Forecasting, IMD Complex, Lodi Road, New Delhi 110003, India. Tel. 91-11-656197/666979 ext. 6023, fax 91-11-6862037.

037%0265/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0377-0265(93)00379-W

2 U.C. Mohanty et al./Dynamics of Atmospheres and Oceans 21 (1994) 1-22

same signature as found during the pre-monsoon month of May. However, the magnitude of the differences shows an overall reduction, with a shift of statistically significant zones from the equatorial region to the northern and eastern sectors of the Arabian Sea.

1. Introduction

The tropical oceans, particularly the Pacific and the Indian Oceans, play a significant role in the establishment and maintenance of the Indian summer (June-August ) monsoon (a seasonal reversal of winds with copious rainfall over most of India and its neighbouring countries). At the same time, it is also well known that the tropical oceans serve as heat reservoirs, and differential heating between the land and the ocean influences the tropical circulation to a large extent. In this context, the tropical oceans which are under the influence of the monsoon (the Arabian Sea, the Bay of Bengal and the Indian Ocean) act as main reservoirs of heat and moisture. They provide the necessary energy to drive the large scale summer monsoon and the associated rainfall over India. These aspects require a bet ter understanding of a i r - sea interaction over the tropical Indian seas on a synoptic, as well as climatic, time-scale. In recent years, a large number of experiments such as I I O E (1964-1965), ISMEX (1973), MONSOON-77 (1977) and MONEX-79 (1979) have been organised to examine the role of the ocean in the maintenance and variability of the monsoon over India. A number of studies have been made to establish the important linkage between a i r - sea fluxes of heat and moisture, and monsoon activity over the Indian sub-continent (Pisharoty, 1965; Das, 1983; Mohanty et al., 1983 and others). Many observational and numerical simulations in recent years support the organisation of an international research programme, the Tropical Ocean and Global Atmosphere (TOGA).

Many studies have been made to establish a possible relationship between sea-surface temperature variations over the Indian seas and monsoon activity over the Indian sub-continent (e.g. Shukla, 1975; Weare, 1979). However, it is strongly believed that it would be more appropriate to establish a link between a i r - sea fluxes and monsoon activity, rather than the sea-surface temperature anomaly and the monsoon, because the interaction between the a tmosphere and the ocean is through the exchange of heat, moisture and momentum at the ocean surface. These fluxes also form the lower boundary conditions which drive large scale ocean-a tmosphere coupled models (Manabe et al., 1975). There has been little work '~o far to establish an association between sea-surface fluxes and the monsoon as the fluxes are not measured directly. The lack of adequate data coverage has also been a handicap. A recent study by Mohanty and Mohan Kumar (1990) with MONEX-79 data showed that a i r - sea fluxes of heat and moisture play an important role in relating the marine boundary layer (MBL) fluxes and the monsoon. On a synoptic scale, we find that an active phase of the summer monsoon is characterised by a net oceanic heat loss, which leads to a positive feedback for the maintenance of deep cumulus convection above the MBL.

Our objective in this study was to examine the impact of a i r - sea fluxes of heat and moisture on the interannual variability of the summer monsoon over the

U.C. Mohanty et al. / Dynamics of Atmospheres and Oceans 21 (1994) 1-22 3

Indian sub-continent. For this purpose, we used the mean monthly marine meteorological fields for a period of 30 years (1950-1979). The variability of the oceanic heat budget components and the surface marine meteorological parameters during contrasting years of summer monsoon activity (large excess rainfall years (flood years) and large rainfall deficient years (drought years)) over the Indian sub-continent are examined.

2. M e t h o d o f c o m p u t a t i o n

The net oceanic heat gain is obtained as a balance of radiative and turbulent heat and moisture fluxes. We express the net oceanic heat budget equation (in W m -2) by

QN = Q R -- QB -- OH - QE (1)

where QN is the net heat flux, QR is the incoming solar irradiance at the sea surface, Qa is the effective outgoing long wave radiation, QH is the sensible heat flux and QE is the latent heat flux. Of these four terms, the short-wave flux contributes significantly to the oceanic heat gain while the other terms contribute to a heat loss from the tropical sea surface. The direct measurement of different components of the oceanic heat budget are meagre. There have been attempts to directly measure some of these components during MONSOON-77 and MONEX- 79 over the Indian seas on synoptic time scales. In this study, we estimate the heat budget components in terms of the observed surface marine meteorological fields. The estimation procedures are discussed in the following section.

2.1. Estimation of incoming solar radiative flux, QR

The solar radiative flux reaching the sea surface constitutes a major source of energy for most of the physical processes over the tropical oceans. As direct estimates of QR are sparse, it is parameterised in terms of the available meteorological parameters. In this study, a method proposed by Reed (1977) was used. The clear sky radiation (Q0) is estimated by a semi-empirical relation proposed by Seckel and Beaudry (1973).

Q0 is then corrected for the transmission of radiation through clouds and the noon altitude of the sun following Reed (1977). The albedo of the sea surface as a function of month and latitude (Payne, 1972) is then used to estimate net incoming solar irradiance at the sea surface.

2.2. Estimation of effective outgoing long-wave radiation, QB

The effective outgoing long-wave radiation from the sea surface depends on the sea-surface temperature, sea-air temperature difference, vapour pressure and the cloud cover of the atmosphere. In general, the effective outgoing long-wave radiative flux does not exhibit much temporal or spatial variability and is very small

4 U.C. Mohanty et al./Dynamics of Atmospheres and Oceans 21 (19q4) 1-22

over the tropical seas. It is parameterised following the semi-empirical method proposed by Giruduk and Malevaski-Holekyich (1973). We find that it gives least errors in the computation of QB during the southwest monsoon season over the Indian seas (Mohanty, 1981; Mohanty and Mohan Kumar, 1991). The effect of cloudiness is included in the estimation of long-wave radiation following the expressions suggested by Egorov (1976).

Based on the special actinometric observations during the lndo-Soviet Monsoon Experiments MONSOON-77 and MONEX-79, the systematic errors in the estimation of long-wave radiative flux are estimated and reduced by a statistical relationship suggested by Mohanty and Mohan Kumar (1991).

2.3. Estimation of sensible and latent heat fluxes QH and QE

The most important transfer processes over the tropical Indian Ocean are the turbulent exchanges of heat and moisture across the air-sea interface. Of the two, the exchange of moisture plays an important role in controlling the net oceanic heat gain and loss because the Bowen's ratio is very small over the tropical oceans. The sensible and latent heat fluxes are computed by a bulk aerodynamic formulation where the fluxes are parameterised in terms of moisture, temperature gradients and the wind velocity at the surface of the MBL. In such procedures, the exchange coefficients play a very important role. In this study, the exchange coefficients are estimated as a function of wind speed and atmospheric stability (Mohanty and Mohan Kumar, 1990). The transfer coefficients are found to be in good agreement with the coefficients obtained by Bunker (1976, 1988) and Kondo and Mirua (1985). A brief outline of the computational procedure is given in Appendix A.

3. Data sources and analysis procedure

The data set for the present study is the Comprehensive Ocean Atmospheric Data Set (COADS), an outcome of a joint venture by the National Oceanic and Atmospheric Administration (NOAA), National Climatic Data Centre (NCDC), Cooperative Institute for Research in Environmental Sciences (CIRES) and the National Center for Atmospheric Research (NCAR) in the USA. This data set is the first of its kind where the most efficient and modern methods were used to compile the atmosphere and ocean parameters over the world oceans. This project was initiated in 1981 (Fletcher et al., 1983), and the quantum of data processed and their spatial distribution are given in a number of COADS documents and publications (Oort et al., 1987).

COADS consists of monthly means of surface marine meteorological fields, compiled and checked for quality, collected by ships and analysed on 2 ° la t i tude/ longitude resolution from 1854 to 1979. The data set for this study has been analysed on a 1 ° latitude/longitude resolution, following Levitus (1982), at the Geophysical Fluid Dynamics Laboratory (GFDL), New Jersey, USA. A large

U.C. Mohanty et al. ~Dynamics of Atmospheres and Oceans 21 (1994) 1-22 5

number of studies have been carried out with this data set to study the influence of oceans on climatic variability (Oort et al., 1987; Rao et al., 1989 and others). The reliability and the quality of COADS are now well established.

The classification of 30 monsoon seasons (1950-1979) in three groups - flood, normal and drought seasons - is based on the seasonal rainfall over India as a whole (following Bhalme and Jadhav, 1984). Any given year having a monsoon rainfall departure of - 1 0 % or more from the long-term mean is classified as a large deficient rainfall year (drought year) and a year with a percentage departure of rainfall greater than + 10% is considered as a large excess rainfall year (flood year). This 10% departure corresponds to about one standard deviation of monsoon rainfall. Of the 30 summer monsoon seasons considered (1950-1979), six were found to be flood years (1956, 1959, 1961, 1970, 1973, 1975), 16 were normal years and eight were drought years (1951, 1952, 1965, 1966, 1968, 1972, 1974, 1979). The domain of the study extends from 24.5°S to 27.5°N and 29.5°E to 109.5°E, which represents the major oceanic regions under the influence of the monsoon. The important geographical regions covered in the analysis area are the Mascarene High, Tropical Indian Ocean, Somali Coast, Arabian Sea and the Bay of Bengal.

The variability of air-sea fluxes and surface marine meteorological fields in May (pre-onset) and June-August (summer monsoon season) for the two categories of monsoon (flood and drought) are examined using COADS. In order to ascertain the statistical significance of the difference between the two extreme categories of the monsoon, the difference fields were subjected to Student's t-test. The statistically significant regions over the Indian seas (above 95% of the confidence limit) are illustrated on the respective difference fields using dot rasters.

4. Results and discussion

In this section, the variability of certain surface meteorological parameters and the components of the oceanic heat budget are studied during the pre-monsoon month of May and the summer monsoon season. The differences between the category of excess rainfall years (flood years) and the category of deficient rainfall years (drought years) are presented in this study. Our aim is to examine the influence of meteorological fields and the oceanic heat budget parameters on the interannual variability of the summer monsoon.

4.1. Sea surface temperature (SST)

A number of observational studies on the variability of SST over the tropics (Weare, 1979; Shukla, 1987; Rao and Goswamy, 1988) have demonstrated that the SST variations over the Arabian Sea and the Bay of Bengal are very small on an interannual scale. The differences of sea-surface temperature between the extreme categories of the summer monsoon (flood-drought) and its corresponding t-distribution are presented in Fig. 1. It shows few significant zones of warmer sea waters

U.C. Mohanty et aL / Dynamics of Atmospheres and Oceans 21 (1994) 1-22

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Fig. 1. Distribution of the anomaly fields of sea surface temperature over the Indian seas between the two extreme categories of the summer monsoon (f lood-drought) . Regions of statistical significance (Student 's t-distribution) with confidence limit above 95% are shaded and dashed contours indicate negative values; (a) for the pre-monsoon month of May; (b) for the summer monsoon season. (Units, 10 - I °C; contour interval, 2 × 10 -1 °C.)

U.C. Mohanty et aL / Dynamics of Atmospheres and Oceans 21 (1994) 1-22 7

a)

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8 u.c. Mohanty et al./Dynamics of Atmospheres and Oceans 21 (1994) 1-22

over the Arabian Sea to the north of 12°N, eastern Indian Ocean (off Indonesia) and the Indian Ocean south of 17°S prior to a flood monsoon season (May). In particular, the significant anomalous warmer oceanic sector observed over the northern Arabian Sea, with a maximum warming of about 0.8°C close to the northwest coast of India, is the main feature of the SST distribution during May. During the same period, a strong nor th-south gradient of surface temperature establishes itself over the Indian sub-continent, with a maximum over northwest India. It is interesting to note that the ocean surface also reflects the same nature of warmer temperature variations as that of the adjoining land mass during the pre-monsoon month of May.

During May, prior to a flood monsoon season, larger oceanic areas marked by colder waters are located over the equatorial Arabian Sea (0°-10°N) and the north Bay of Bengal. The anomalous cooling over the Indian seas is mainly attributed to the intense latent heat transfer and upwelling under the influence of stronger surface winds over the equatorial belt of the Arabian Sea prior to the onset of monsoon over India. However, the anomalous cooling effect of the equatorial sea does not attain statistical significance.

With the establishment of the summer monsoon, a significant change in the SST anomaly over the entire Arabian Sea and the Bay of Bengal, characterised by sea-surface cooling, is observed (Fig. l(b)). However, the SST anomalies during the summer monsoon season attain statistical significance only over a small region adjoining the east coast of India and Madagascar. The negative anomaly of the SST (cooling) all over the Indian seas is consistent with strong low-level flow over the region with enhanced evaporation leading to surface cooling during the flood monsoon season over India.

4.2. Surface wind

The surface wind anomaly distribution between the two extreme categories of the summer monsoon (f lood-drought) is presented in Fig. 2. During May, a strong positive surface wind anomaly is observed over the entire equatorial belt of the Arabian Sea (0°-15°N). The pre-monsoon month of a flood monsoon season shows a maximum positive surface wind anomaly of about 4 m s - 1 over the southwestern sector of the Arabian Sea. During the same period, a positive wind anomaly of the order of 1-2 m s- 1 is also noted over the head Bay of Bengal. It may be noted that in the month preceding a large excess rainfall year, the strong positive anomaly zone of surface wind over the equatorial Arabian Sea is associated with active convective processes under the influence of intense turbulent transfer of moisture leading to surface cooling (Fig. 1), Furthermore, the surface wind over the easterly trade wind regions of the equatorial Indian Ocean (0°-15°S) is found to be relatively stronger during the May preceding a flood monsoon season. All these three positive surface wind anomaly zones are statistically significant (Fig. 2(a)).

The positive surface wind anomaly pattern which is the characteristic feature of the pre-monsoon circulation over the Indian seas is maintained during the summer monsoon season as well. The results are consistent with the findings of Cadet and

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Diehl (1984) who suggested a reduction of surface wind over the Indian seas in a drought monsoon as opposed to a substantial increase of surface winds in a flood monsoon season. A zone of positive wind anomaly with the desired level of statistical significance over the Indian ocean off the east coast of Africa suggests a strong cross-equatorial flow during the flood monsoon seasons.

It is generally found that strong cross-equatorial flow off the Somalia coast over the Arabian Sea is well established prior to the advancement of the summer monsoon (about 2 weeks) over the Indian sub-continent (Mohanty et al., 1985). The Bay branch of the summer monsoon flow also covers the southern, central and eastern regions of the Bay of Bengal by the middle of May, Satellite data also confirm the fact that about 10-15 days prior to the onset of monsoon over the southwest coast of India, intense cloudiness and disturbed weather conditions prevail over the equatorial Indian seas (Krishnamurti et al., 1979; Mohanty et al., 1983).

It is interesting to note that a zone of positive surface wind anomaly over the eastern Arabian Sea off the entire west coast of India is the most prominent zone of wind anomaly having the required level of statistical significance during the summer monsoon. This zone plays an important role in the development of intense convective activity and clouding.

4.3. Cloud cover

The distribution of cloud cover anomalies during May and the summer monsoon season follow the same pat tern as the surface winds for the corresponding periods over the Indian seas. This is because the genesis of cloud cover over the warm tropical oceans is mainly due to convective activity triggered by strong surface winds, and turbulent transport and frictional convergence of moisture in the marine boundary layer.

We find that the pre-monsoon month of May, preceding a flood monsoon season, is characterised by a significant positive anomaly of cloud cover extending all over the Arabian Sea (with a maximum over the south central region) and the Bay of Bengal. Both the zones of positive cloud cover anomalies are found to be statistically significant (Fig. 3(a)).

With the onset of the summer monsoon, the differences of cloud cover between flood and drought seasons depict a positive anomaly all over the Bay of Bengal and the Arabian Sea. The magnitudes of anomalies are substantially reduced and small localised maxima are located off the coast of India, northern Arabian Sea and the oceanic region adjoining Sri Lanka (Fig. 3(b)).

4.4. Solar radiative flux

The distribution of short-wave fluxes during May, prior to the summer monsoon, and the subsequent monsoon season exhibit an interesting pat tern irrespective of the monsoon activity over India (figures not presented), During May, a

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strong north-south gradient of fluxes is found over the Arabian Sea and the Bay of Bengal. With the arrival of the summer monsoon, the orientation of the distribution of short-wave radiative fluxes changes to an east-west direction. This change in the distribution of short-wave fluxes is an outcome of the prevailing east-west oriented cloud cover in association with the southwesterly monsoon flow.

The short-wave radiation flux anomaly for May and the subsequent monsoon, along with the corresponding significant student's t-test values, are illustrated in Fig. 4. During the pre-monsoon month of May, the flux anomalies show a clear reduction in incoming short-wave flux by some 20-30 W m -2 prior to a flood- monsoon season. During the summer monsoon, a similar pattern of negative anomaly is also observed over the Arabian Sea and the Bay of Bengal. The zones of maxima in short-wave flux anomalies observed over the oceanic regions are found to be statistically significant. They indicate a pattern similar to the cloud cover which plays a significant role in modulating the incoming solar radiation.

4.5. La ten t heat f lux

In May, a maximum zone of latent heat flux is found over the southwestern sector (0°-10°N) of the Arabian Sea. It moves to the northeastern Arabian Sea with the establishment of the monsoon irrespective of the intensity of monsoon activity over India (figures not presented). This shift in the zone of maximum latent heat flux is in agreement with the movement of a zone of maximum low-level wind over the same region in association with the progress of the monsoon.

The latent heat flux anomalies between large excess (flood) and large deficient (drought) monsoons and their corresponding t-distributions for May are presented in Fig. 5. As one might expect, the zones of flux anomalies and their statistical significance are in good agreement with the corresponding surface wind anomalies. The distribution of latent heat flux differences show a region of significant positive anomaly (maximum of 100 W m -2) over the southwestern sector of the Arabian Sea (Fig. 5(a)). Although the positive and negative anomalies of latent heat and short-wave fluxes are comparable over most parts of the Indian seas, it may be noted that the latent heat flux anomaly over southwest Arabian Sea during May is abnormally large (about three times the short-wave flux). Considering such large magnitude of latent heat flux anomalies, and their statistical significance in May, the appearance of this anomaly may be treated as an advance indicator of the possible behaviour of the subsequent monsoon season.

During the summer monsoon, a significant positive anomaly in the latent heat flux is found over the northeastern Arabian Sea off the west coast of India and over the south Indian Ocean adjoining Madagascar (Fig. 5(b)). The other zones of latent heat flux anomalies (both positive and negative) with magnitudes of 10-20 W m -2 do not attain the desired level of statistical significance. During the monsoon season, negative anomaly zones of latent heat flux over the Arabian Sea and the Bay of Bengal coincide with zones of upwelling and sea-surface cooling. These results do not depict any statistical significance.

14 U.C. Mohanty et al. / Dynamics of Atmospheres and Oceans 21 (1994) 1-22

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U.C. Mohanty et aL ~Dynamics of Atmospheres and Oceans 21 (1994) 1-22 15

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4.6. Sensible heat and long-wave radiative fluxes

The magnitudes of the other two terms in the oceanic heat budget, sensible heat and outgoing long-wave radiation, are found to be considerably less than the incoming short-wave radiation and the latent heat fluxes. Both terms contribute towards the oceanic heat loss such as the latent heat flux. It has been noted that the combined contribution of these two terms towards the oceanic heat loss does not exceed 10%. Also, their geographical distribution, in general, appears to be very similar to that of the SST. Their magnitudes do not change significantly from one category of the monsoon to another. Consequently, these components of the oceanic heat budget are not discussed in this study, though they are used for the estimation of the net oceanic heat flux.

4. 7. Net oceanic heat f lux

As seen in the previous section, contributions of sensible heat and outgoing long-wave radiation are very small and do not vary much from one category of the monsoon to another. Thus, the net heat flux over the ocean mainly reflects a balance between short-wave radiation and latent heat flux. Over the tropical oceans, the latent heat flux represents a heat loss from the ocean surface while the incoming short-wave radiation represents a heat gain. To a large extent, these two terms compensate one another during the summer monsoon season, m negative magnitude of the net oceanic heat flux implies a dominance of the latent heat flux over incoming short-wave radiation. Under such conditions, large evaporation from the tropical oceans leads to strong moisture convergence and deep cumulus convection. This leads to an overall increase in cloudiness and substantial reduction of incoming short-wave flux. On the other hand, a positive net oceanic flux represents less cloud cover with increased short-wave flux and less evaporation leading to clear undisturbed weather conditions. Therefore, the net oceanic heat flux provides a measure for convective activity associated with the summer monsoon events.

The distributions of net heat flux for May and the summer monsoon seasons for two extreme categories of the monsoon ( f lood/drought ) are presented in Fig. 6. We see that the equatorial Arabian Sea (0°-10°N) is characterised by a net heat loss from the ocean (with a maximum over the southwestern sector) during the pre-monsoon month of May preceding a flood monsoon season. For a drought situation, the zone of heat loss observed over the equatorial Arabian Sea is replaced by a zone of heat gain. However, over the Bay of Bengal, a zone of heat loss from the ocean is observed irrespective of the nature of subsequent monsoon over India.

A northward progression of the region of net heat loss from the sea surface is observed over the Arabian Sea during the summer monsoon season (Figs. 6(c) and 6(d)). It is also observed that the flood monsoon seasons are associated with a large heat loss from the ocean to the atmosphere over the Arabian Sea and the Bay of Bengal. However, heat loss is less for the drought monsoons.

U.C. Mohanty et aL / Dynamics of Atmospheres and Oceans 21 (1994) 1-22 17

a)

~o~n

~O~n

I O°S

2o~s

3~E 5gE 75E 95E

"-- 20N " . ,, (

° . : °

- !i: ~ 0 ~ \ I

3 5°E 5 5°E 7 5 °E 9 5°E

b)

~o~n

~n

o o

10°S

2oOs !

3gE 5gE 7gE ggE '

~:':,:.

- ' " ~ ".':~:':' - ' - n :'.-:..

LIOJ ,/ ,.., \ /. .... , ....

--, / , - - - - " / f -30-...' .',.,~x,'F~ \ "% ( - -~0 . , / / , - , ~ . ; / / / ~ x , ~ ; . - ~

"~ . ~. ~ , ~ ; ~ - , ,( \ i . . x -II1 ( ~ 111(('4.o , ;

, . ~ [ " , / "\ X x " - . ~ I \ v//-/-/-/-/-/-/-/-/-/~?n~\2~('~,g-~-'n~-'N \ / ~ , ' , ( / "', , , .... ""I ~ I ~ ~ ' , : : ~ - . ~ )

35°E 5 5°E 7 5~ 95°E

2o~n

IO~N

0 o

0°s

Fig. 7. As Fig. 1, but for the net oceanic heat flux. (Units, W m-2 ; contour interval, (a) 20 W m-2 ; (b) 10 W m-2 . )

18 u.c. Mohanty et al. / Dynamics of Atmospheres and Oceans 21 (1994) 1-22

The distribution of net heat flux differences between the extreme categories of the summer monsoon is illustrated in Fig. 7. In May, a strong negative anomaly in oceanic heat gain/loss is observed over the Bay of Bengal, the Arabian Sea and the Indian Ocean. It implies that during the pre-monsoon month of May, preceding a flood monsoon, the oceanic region under the influence of the strong winds loses a considerable amount of heat by intense convective activity (enhancement of latent heat flux and reduction of solar insolation). The zone of maximum heat loss (about 100 W m -2) is observed right over the maximum latent heat flux anomaly (Fig. 5(a)). It is the main geographical region having the required level of statistical significance (Fig. 7(a)). As stated previously, this feature provides a useful indication of the subsequent mean seasonal monsoon over the entire Indian sub-continent ( f lood/drought) .

During the monsoon season, the net heat flux anomalies remain the same as in May, except over the upwelling region of the Arabian Sea off the coast of Somalia. In agreement with the patterns of other major oceanic heat flux components, the zone of minimum anomaly moves to the northern and eastern sectors of the Arabian Sea and those are the regions of statistical significance during the summer monsoon (Fig. 7(b)).

5. Conclusions

The results of our study may be summarised as follows: (1) prior to a large excess rainfall season over India, a significantly warmer

oceanic region exists over the north Arabian Sea close to the northwest coast of India during May. However, during the monsoon season, SST anomalies do not display any significant variations between the two extreme categories of the monsoon (f lood/drought) ;

(2) the surface wind fields depict a significant zone of stronger winds over the southwestern sector of the Arabian Sea during May preceding a flood monsoon. Generally, the cloud cover variations are found to follow the distribution of surface wind over the Indian seas. This may be attributed to the fact that anomalously stronger surface winds over the warm oceanic regions are responsible for intense turbulent transport of moisture and frictional convergence in the planetary boundary layer which lead to the development of convective activity and the subsequent formation of clouds;

(3) excessive cloud cover over the regions of stronger surface winds is characterised by a significant reduction in incoming solar radiation at the sea surface and anomalously high evaporation to the overlying atmosphere. These two effects contribute in the same sense to deplete oceanic heat content, thus favouring surface cooling, as the effect of evaporation is larger than that of the radiation input during the May prior to a flood monsoon over India. Thus, the characteristic anomalous features identified from our analysis of marine surface meteorological parameters and the oceanic heat budget during May provide a useful qualitative indication of the subsequent mean monsoon activity over the Indian sub-continent;

u.c. Mohanty et al. ~Dynamics of Atmospheres and Oceans 21 (1994) 1-22 19

(4) the results also reveal that during the monsoon season, the anomalies in the basic marine meteorological fields and oceanic heat fluxes are found to maintain the same signature as during May. However, the magnitudes of the anomalies show an overall reduction. Furthermore, with the progress of the monsoon to the northeastern Arabian Sea, a shift in the statistically significant region from the equatorial Arabian Sea to its northern and eastern sectors is observed.

Acknowledgements

We are indeed grateful to Prof. A.H. Oort of the Climate Dynamics Group of the Geophysical Fluid Dynamics Laboratory (GFDL), Princeton University, N J, USA, for providing the analysed Comprehensive Ocean Atmospheric Data Sets (COADS) used in this study. We express our sincere thanks to Prof. P.K. Das for his enlightening discussions and critical comments. We also thank the anonymous reviewers for their useful comments which have improved the quality of the manuscript.

Appendix A: Oceanic heat budget computation

The net oceanic heat budget equation (in W m -2) can be written as

QN = QR - Qa - QH - Q~ (1)

(a) Estimation o f incoming solar radiative flux, QR

Q0 is computed as a function of geographical and astronomical factors like latitude of the place and time of the year following Seckel and Beaudry (1973) in the following manner:

Q0 =A0 +A1 cos(d,) + B 1 sin(~b) + A 2 cos(24,) + B E sin(E~b)

where:

A 0 -- - 15.82 + 326.87 c o s ( L ) ;

A 1 = 9.63 + 192.44 cos( L + ~-/2) ;

A 2 = - 0 . 6 4 + 2*7.80 s in(L - ~ ' /4 ) cos (L - zr /4) ;

B x -- - 3.27 + 108.70 s in (L) ;

B E = - 0 . 5 0 + 14.42 cos (2 (L - r r / 3 6 ) ) ;

4' = (JD-21) * 360/365 (in radians);

JD is the julian day;

L is the latitude of a place (in radians).


Q0 in w m-2, is then corrected for the transmission of radiation through clouds and noon altitude of the sun following Reed (1977):

QR = Qo( 1 - 0.62C + 0.0019H)(1 - A ) (2)

where C is the cloud amount in fraction, H is the noon altitude of the sun in degrees and A is the albedo of the sea surface. The noon altitude of the sun is obtained as

sin(H) = sin(L) sin(d) + cos(L) cos(d) (3)

where L is the latitude of a place and d is the declination of the Sun. The declination of the sun is obtained as a function of the julian day (JD) in the following manner

d = -23.45 cos(T)

where T = (JD × 360)/365 (in radians). The albedo of the sea surface, A, as a function of month and latitude is estimated following Payne (1972).

(b) Estimation of effective outgoing long-wave radiation, QB

In general, the effective outgoing long-wave radiation flux does not exhibit much temporal and spatial variabilities and is a very small quantity over the tropical seas. It is parameterised following Giruduk and Malevaski-Holekyich (1973) which is found to give least errors in the computation of QaR during the southwest monsoon season over the Indian Seas (Mohanty, 1981; Mohanty and Mohan Kumar, 1991).

Based on the special actinometric observations collected during the Indo-Soviet Monsoon Experiments during MONSOON-77 and MONEX-79, the systematic errors in the estimation of long-wave radiative flux are reduced between the computed values of QBR and the measured values by establishing a statistical relationship (Mohanty and Mohan Kumar, 1990) as given below.

QB = 0.0308 + 0.3174QB R (4)

The small values in the regression coefficients in Eqn. (4) suggest that QBR is very close to QB and thus the empirical relationship used for the estimation of QBR are fairly accurate. However, considerable reduction of systematic errors in the estimation of long-wave flux can be achieved through the use of Eqn. (4) to overcome certain inadequacies in the surface observations (Mohanty and Mohan Kumar, 1991).

(c) Estimation of sensible and latent heat fluxes, Qt4 and QE

The sensible and latent heat fluxes are computed based on the bulk aerodynamic formulation where the fluxes are parameterised in terms of moisture and


t empera ture gradients and wind velocity at the surface of the MBL. The latent heat flux (QE) is given by

QE = p L C E U [ a ( Ts) - qa] (5)

and the sensible heat exchange across the a i r - sea interface is expressed as

QH = p C p C H U [ T s - Ta] (6)

where U is the wind speed. In this study, both the exchange coefficients C E and C n are assumed to be same. For their computation, the following formulations based on wind speed and atmospheric stability are used (Mohanty and Mohan Kumar, 1990). For U < 5 a n d A T < I

103Cn -- 0.926 + 0.682AT + 0.046U - 0.093AT 2 + 0.00074U 2

For U < 5 and 1 < A T < 9

103CH = 1.751 + 0 . 1 0 2 A T - 0 . 1 5 7 U - 0.007ATU - 0.003AT 2 + 0.011U 2

For 5 < U < 9 and 1 < A T < 9

103CH ---- 1.085 + 0 . 1 4 5 A T - 0.007U

For 9 < U < 15 and 1 < A T < 9

103CH ---- 1.03 + 0.067AT + 0.025U - 0.003ATU - 0.002AT 2 - 0.00021U 2

where AT is the effective tempera ture expressed as (T s - Ta)(1 + 0.07/B), which gives a measure of atmospheric stability, and B is the Bowen's ratio given by B = C p P ( T s - Ta) /O.622L(e s - ea).

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