Water vapour transport over the Indian Ocean during summer 1975

Abstract

Water vapour transport over the Indian Ocean during the 1975 Indian summer monsoon is investigated. The main data set used in this study is constituted of twice-daily surface assimilated meteorological fields determined on a 2.5 degree long grid from an objective assimilation of surface data over the Indian ocean. Upper air data are also used. Daily evolution of surface water vapour fluxes across different sections are presented. In the Arabian Sea, the onset of the monsoon is characterized by a large increase of the surface water vapour fluxes. The main feature of the fluxes is the existence of an oscillation with a fundamental period of 10 & 2 days. The analysis of correlation coefficients between fluxes across sections in the Arabian Sea suggests that periodic variations of water vapour fluxes result from downstream forcing. Mean monthly water vapour budgets have been estimated over the Arabian Sea and the Bay of Bengal. The results show that about 70% of the water vapour that crosses the western coast of India comes from the Southern Hemisphere whereas 30% is picked up by evaporation over the Arabian ocean. In the Bay of Bengal, water vapour picked up by evaporation over the ocean seems to be largely used for local weather development.