On the sea surface temperature high in the Lakshadweep Sea before the onset of the southwest monsoon

The north Indian Ocean becomes the warmest area of the world oceans prior t o the onset of southwest monsoon in June. During this period a zonal band o f high sea surface temperature (SST), the "thermal equator" (TE), moves ove r this region concurrently with the Intertropical Convergence Zone (ITCZ). Using a weekly SST data set, we show that another SST high develops off sou thwest India in the Lakshadweep Sea in March, well before the TE moves in t o the area, and that it continues to retain its identity until the onset of monsoon. The SST high has its genesis about 6 months earlier in the Bay of Bengal. The collapse of the southwest monsoon in October and the onset of the northeast monsoon trigger downwelling coastal Kelvin waves that propaga te along the periphery of the Bay of Bengal, forcing an equatorward East In dia Coastal Current, which brings low-salinity water from the bay to the so utheastern Arabian Sea during the northeast monsoon (November-January). As the Kelvin waves propagate poleward along the west coast of India after tur ning around Sri Lanka, they radiate downwelling Rossby waves that produce a "high" in sea level off southwest India. The downwelling and the surface l ayer of low-salinity water provide a breeding ground for the formation of a SST high in January. By March, with the increase in solar insolation due t o the northward march of the Sun and the deep stable surface layer, the hig h reaches a mature phase clearly evident in the Lakshadweep Sea. By May, wh en the thermal equator and ITCZ move over the region, the high can be seen embedded in the TE. We speculate that at this time the high helps in produc ing conditions that are conducive for genesis of the monsoon onset vortex.