Water Vapor Transport over the Indian Ocean during the 1979 Summer Monsoon. Part I: Water Vapor Fluxes

Abstract

Water vapor transport over the Indian Ocean during the 1979 summer monsoon is studied. The analysis is based on wind fields from the European Centre for Medium Range Weather Forecasts and humidity fields derived from a three-layer precipitable water dataset. Fields of zonal and meridional water vapor fluxes show significant variations over the north Indian Ocean in association with the different phases of the 1979 monsoon. Whereas after the onset, the cross-equatorial water vapor flux west of 50°E does not vary much; it undergoes significant fluctuations east of that longitude. The bulk of water vapor crossing the western coast of India comes from the Southern Hemisphere. The latitude band between 10° and 20°S appears as a major source of moisture during the northern summer. The major moisture supply for the western coast of Burma and Thailand is advected over the Bay from the Arabian Sea branch of the monsoon. The water vapor flux across the west coast of India undergoes large amplitude variations in relation with the active/break cycle of the 1979 monsoon (onset, active, break and revival periods). During active periods, the moist flow over the Arabian Sea strengthens and deepens. The water vapor flux across the west coast of India is well related to rainfall along the coast. Ale early retreat of the 1979 monsoon is associated with a decreasing trend in moisture transport over the Arabian Sea. In the Bay of Bengal, the cross-equatorial flux is not affected by the break/active cycle of the monsoon. There are strong surges of northward flux into the Bay. Some of them are related to the formation of Bay depressions.