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

Abstract 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°C is manifested in the south‐eastem 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.