Water Vapor Transport over the Indian Ocean during the 1979 Summer Monsoon. Part II: Water Vapor Budgets

Abstract

In this second part of the paper, moisture budgets over the Arabian Sea and the Bay of Bengal are investigated for the 1979 summer monsoon season. Over the Arabian Sea the different terms of the moisture balance equation, except evaporation, strongly fluctuate depending on the activity of the monsoon. The relative contribution to the monsoon moisture supply by water vapor transport across the equator and Arabian Sea evaporation varies as the monsoon intensity changes from active through break and back to revival stages. However. it is shown that water vapor from the Southern Hemisphere is the major source of moisture for Indian rainfall. Total evaporation during the active period following the onset of the monsoon is found to be 30?40% of the total eastward flux across the west coast of India. This ratio increases to 40?45% during break condition but falls below 20% during a revival phase. These moisture budgets also show that convergence of water vapor flux is limited to the eastern part of the Arabian Sea whereas evaporation exceeds precipitation in the western Arabian Sea Moisture budgets over the Bay of Bengal depend strongly on the monsoon intensity and the amount of moisture advected across the western coast of India and into the Bay of Bengal by the monsoon circulation. Moisture supply from the Southern Hemisphere via cross-equatorial flux at the longitude of the Bay of Bengal is very weak. Compared to weak monsoon periods a much larger percentage of the water vapor supplied by evaporation and boundary fluxes is transported towards Burma and Malaysia during active monsoon periods, fueling the heavy rainfall them.