Conditions leading to monsoon onset over Kerala and the associated Hadley cell

Abstract

Monsoon onset over Kerala (MOK) is an important event in relation to the Asian Summer Monsoon and the global atmosphere and Ocean. It marks the beginning of an intense and large convective heat source north of the equator over the tropical Indian and west Pacific oceans and an intense low level jet stream that feeds moisture to the heat source, both having a life span of about 100 days from MOK.  Using satellite data we are now able to monitor the slow evolution of this heat source which first forms near the equator over an area with large SST gradient (in the south-north direction) and slowly moves towards the warm pool over the southeast Arabian Sea.

A Monsoon Hadley cell (MHC) is associated with this large convective heat source which is particularly strong over the south-east Arabian Sea at MOK. The ascending branch of the MHC is over this heat source and its descending branch is over the subtropical south Indian ocean. The low level branch of the MHC called V850 is a strong meridional southerly flow at 850 hPa crossing the equator between the longitudes 35° E and 55° E. The upper level branch V200 is a meridional northerly flow at 200 hPa over the south Indian ocean between longitudes 45° E and 75° E. The intensity of the MCH taken as (V850 -V200) is found to increase rapidly during the 10 day period prior to MOK.