The annual mean precipitation values used in the present study are extracted from the INSAT-IB derived precipitation values for the year 1987 over the tropical Indian Ocean using the Very High Resolution Radiometer (VHRR) sensor working in the range 10.5-12.5 um. We assume that there will not be large inter annual variations in the oceanic precipitation for the study area. The major reason for using the satellite derived precipitation is the non-availability of oceanic precipitation values for the tropical Indian ocean. Rao et al. (1981): Vinayachandran and Ramesh Kumar (1989) have validated the satellite derived precipitation values with the observed island station data, and have found that they differ by about 50% and 25% respectively.

Results and Discussions
Since the bulk of the moisture exists in the lower part of the atmosphere, we can interpret Fig. 1 in terms of the column divergence/convergence

Figure 1 presents the moisture flux divergence/convergence/at the sea surface over the tropical Indian Ocean. The most interesting feature of the above figure is the region of large scale convergence (dashed area) in the eastern Indian Ocean, especially Bay of Bengal area. This indicates that this region is highly conductive for large-scale connective activity like the formation of monsoon depressions and tropical cyclones round the year. From this it can be seen that the evaporation exceeds precipitation over all the tropical Indian Ocean except the eastern half of it. Thus it further emphasizes the role of the Arabian Sea and the cross-equatorial moisture flux than the moisture from the Bay of Bengal region for the summer monsoon rainfall over the Indian subcontinent.

Table 1 gives a comparison of the water vapor flux values for the AS during several contrasting monsoon seasons. An analysis of Table 1 shows that the moisture flux over the AS exhibits large inter seasonal and inter annual variations. The values for 1975 and 1979 were obtained using the upper air data, where as for the rest of the years it is obtained as the difference between the E-P estimates. Hence the large variability in the moisture flux divergence values can be attributed to the precipitation fluctuations or the errors in the precipitation estimation.

Conclusion

1. The annual mean picture of the moisture flux divergence shows that the Bay of Bengal region is highly conductive for large scale convergence.



2. The mean moisture flux divergence values for the year 1987 are -0.02 x 1010, 2.25 x 1010, 0.70 x 1010 and 0.44 x 1010 tons/day respectively for the months June to September.



3. The study further points to important role of AS and the southern Indian Ocean moisture source for the monsoon rainfall than the Bay of Bengal region.

Acknowledgements
The authors are grateful to the Director, N.I.O. and Dr. J.S. Sastry, Deputy Director and Head, Physical Oceanography Division for their keen interest and encouragement.

References

1. Cadet, D.L. and Greco, S., 1987. Water vapor transport over the Indian Ocean during 1979 summer monsoon. Part 1 : Water vapor fluxes. Mon. Wea. Rev., 115, 653-663.



2. Howland, M.R. and Sikdar, D.N., 1983. The moisture budget over the northeastern Arabian Sea during the premonsoon and monsoon onset 1979. Mon. Wea. Rev., 111, 2255-2268.



3. Murakami, T., Nakazowa, T. and He, T.,1984. On the 40-5- day oscillations during the monsoon during the northern hemispheric Japan, 62, 469-484.



4. Rao, G.V., Schaubb, Jr., W.R. and Peutz, J., 1981. Evaporation and precipitation over the Arabian Sea during several monsoon seasons. Mon. Wea. Rev., 109, 364-370.



5. Rasmusson, E., 1972. The general circulation of the Tropical Atmosphere. R.E. Newell, J.W. Kidson, D.G. Vincent and G.J. Boer, Eds., The MIT Press.



6. Sedhuram, Y. and Ramesh Kumar, M.R., 1988. Does evaporation over the Arabian Sea play a crucial role in moisture transport across the west coast of India during an active monsoon period ? Mon. Wea.Rev.,307-312.



7. Vinayachandran, P.N> and Ramesh Kumar, M.R., 1989. Water Vapour flux divergence over the Arabian Sea during 1987 summer monsoon using INSAT-IB data. (Submitted to Boundary Layer Meteorology.

Table-1 A comparison of water vapour flux values over the Arabian Sea during contrasting monsoon years. Unit: 1010 tons/day

Month/Year	1964	1973	1974	1975	1979	1987
June	0.51	0.36	1.90	3.10	0.02	-0.02
July	0.48	1.38	2.40	3.00	0.05	2.55
August	-0.27	------	2.31	3.50	0.46	0.70
September	0.82	------	------	2.80	-0.23	0.44

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