Space-Time Evolution of Meteorological Features Associated with the Onset of Indian Summer Monsoon

Abstract

To study the climatological structure of the atmospheric fields during the onset phase of the Indian summer monsoon, a composite analysis of different meteorological parameters over Indian stations is carried out. The composites are constructed relative to a uniform set of onset dates over south Kerala. Over the peninsular Indian stations, the rainfall composites show sudden and sharp increases with onset except in the case of east coast stations, where rainfall does not substantially change with the onset of the summer monsoon. The composite wind analysis demonstrates how the upper-tropospheric subtropical westerlies weaken and shift poleward and the tropical easterlies strengthen and spread north with the onset of the monsoon. The onset vortex that takes the monsoon northward along the west coast in many years is clearly discernible between 600 and 400 hPa in the composite streamline charts. The relative humidity builds up suddenly in the vertical a few days before the onset at the respective stations. The vertically integrated zonal moisture transport at individual stations over the peninsula increases sharply with respect to the south Kerala onset, with appropriate lag in time. The composite outgoing longwave radiation fields over the north Indian Ocean show rapid buildup of convective activity over the southeast Arabian Sea and east Bay of Bengal with the approach of the monsoon.