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ABSTRACT

In this paper, we suggest that positive Indian Ocean dipole events could be triggered by the occurrence of severe cyclones over the Bay of Bengal during April–May. All positive Indian Ocean dipole events during 1958–2003 are preceded by at least one such severe cyclone. Severe cyclones over the Bay of Bengal strengthen the meridional pressure gradient across the eastern equatorial Indian Ocean (EEIO) and hence lead to the intensification of the upwelling favourable southeasterlies along the Sumatra coast. Severe cyclones can also lead to a decrease in the integrated water vapour content and suppress convection over the EEIO. We suggest that the suppression of convection over the EEIO in turn, leads to the enhancement of convection over the western equatorial Indian Ocean (WEIO) and hence to the weakening of the westerlies along the central equatorial Indian Ocean (CEIO). This can lead to a positive feedback between suppression of convection over the EEIO and enhancement of the convergence and convection over the WEIO. If this positive feedback continues until the winds over the CEIO become easterlies, the convection over the EEIO remains suppressed for a period much longer than the synoptic scale. The strong upwelling caused by the easterlies along the equator and southeasterlies along the Sumatra coast decreases the sea surface temperature of the EEIO very rapidly and a positive dipole event gets triggered.