On the possible mechanisms of the evolution of a mini-warm pool during the pre-summer monsoon season and the genesis of onset vortex in the south-eastern Arabian Sea

During the pre-summer monsoon season (February-May), the near-surface waters in the Arabian Sea progressively warm up and a mini-warm pool with a core >30 degrees C is manifested in the south-eastern region. The possible mechanisms for the observed seasonal build-up of this mini-warm pool are examined, utilizing all the available monthly mean climatologies of surface wind field, surface heat fluxes, near-surface thermohaline fields, near-surface circulation, and mean sea level as monitored by satellites and by some of the recent model solutions on the Arabian Sea circulation. During winter (November-February), the equatorward-flowing East India Coastal Current in the western Bay of Bengal and westward-flowing North Equatorial Current in the southern Bay bring low-saline waters into the south-eastern Arabian Sea, causing a haline stratification within the near-surface isothermal layer. During December-April, the positive surface-wind-stress curl and the associated Ekman divergence shoals the pycnocline. A south-westward propagating mode-2 Rossby wave from off south-west India seen in satellite-derived mean sea level and model solutions also modulates the underlying pycnocline. During the pre-summer monsoon season, under clear skies and light wind conditions, the radiative heat input overwhelms turbulent heat losses at the air-sea interface, and the net surplus heat energy is absorbed in a shallow haline stratified near-surface layer, resulting in the formation of the observed mini-warm pool. An examination of historical data on the genesis of monsoon onset vortices reveals that on most occasions the genesis has occurred over this mini-warm pool region. Evidence for the geographic coincidence in the occurrence of the genesis of onset vortex and the sea surface temperature maxima during individual years of a three decade period (1961-90) is presented.