The summer monsoon onset process over South Asia and an objective method for the date of monsoon onset over Kerala

Abstract

Eight pentads before the monsoon onset over Kerala (MOK), a spatially large area of deep convection formed near the equator south of the Bay of Bengal, which moved to Southeast Asia marking the onset of the South China Sea monsoon (SCSM) for many years. Three pentads before MOK, a similar area of convection formed near the equator south of the Arabian Sea. This heat source and the associated cross-equatorial low-level jet stream (LLJ) grew steadily in strength while moving north and at MOK the convective heat source passed through Kerala latitudes and the core of a well developed LLJ was located just south of Kerala. Eight pentads before MOK a warm pool was located over central Bay of Bengal and the area of active convection formed to its south near the equator in the region of large sea surface temperature (SST) gradient. Three pentads before MOK when the Bay of Bengal SST had cooled, a warm pool formed over central Arabian Sea and an active convection area was located south of it, also in the region of large SST gradient. A three-step method for objectively defining MOK has been developed in this paper. In step 1 of this operationally usable method, the date on which the zonal wind of 850 hPa, averaged over a box bounded by latitudes 5°N and 10°N and longitudes 70°E and 85°E, reached 6 m/s at 600 hPa is taken as the tentative date of MOK. Steps 2 and 3 checked whether the date thus chosen was a bogus monsoon onset or not and whether on that date there was widespread convection (low OLR) around Kerala, which moved north from the equatorial region. Copyright © 2006 Royal Meteorological Society.