Monsoon Response of the Sea around Sri Lanka: Generation of Thermal Domesand Anticyclonic Vortices

Abstract

Results from an ocean general circulation model are used to study the response of the oceanic region surrounding Sri Lanka to monsoonal winds. East of Sri Lanka, a cold dome (Sri Lanka dome, SLD) develops during the southwest monsoon (SWM) in response to cyclonic curl in the local wind field. The dome decays after September due to the arrival of a long Rossby wave, associated with the reflection of the spring Wyrtki jet at the eastern boundary of the ocean. East of the SLD an anticyclonic eddy exists that is in intermediate geostrophic (IG) balance. North of Sri Lanka a cold dome (Bay of Bengal dome) develops after the SWM associated with a cyclonic gyre forced by Ekman pumping. The source of cold water of the Bay of Bengal dome is traced back to the SLD and upwelling zone along the east coast of India. South of Sri Lanka a major part of the Southwest Monsoon Current (SMC) turns northeastward and flows into the Bay of Bengal. The part that flows eastward terminates at progressively western longitudes as the season progresses. This termination and the shallowness of the SMC is due to a Rossby wave generated near the eastern boundary by weakening of the spring Wyrtki jet and anticyclonic wind stress curl. This Rossby wave follows the one associated with the spring Wyrtki jet and has dominant velocities toward southwest. A large anticyclonic vortex, embedded in the SMC, results from the geostrophic adjustment process for the surface water converged by the long Rossby wave and the eastward zonal current. Energy analysis of this anticyclonic vortex as well as the IG eddy east of the SLD shows direct conversion from mean kinetic energy to eddy kinetic energy suggesting that barotropic instability is the mechanism that leads to eddy generation. This study suggests two links that allow exchange between the Bay of Bengal and the rest of the Indian Ocean: The first is the SMC, which is an open ocean current, and the second is the equatorward East India Coastal Current during November?January, which is closely attached to the coast.