Interdecadal Shift in Tropical Cyclone Genesis Between the Arabian Sea and the Bay of Bengal at the Post‐Monsoon Season

ABSTRACT

Interdecadal variations in tropical cyclone genesis (TCG) over the Arabian Sea (AS) and Bay of Bengal (BoB) during the post‐monsoon season are investigated for 1975–2021. The results reveal a marked interdecadal transition in the correlation of TCG between the two basins. During 1975–2004 (Period 1, P1), the correlation was predominantly negative, but became positive during 2005–2021 (Period 2, P2). Specifically, the interdecadal increase in TCG over the AS and the relatively stable TCG over the BoB during P2 are associated with this interdecadal correlation shift. The Dynamic Genesis Potential Index (DGPI) indicates that intensified upward motion, enhanced mid‐level vorticity, and sea surface temperature (SST) warming contributed to the increase in TCG over the AS during P2. Meanwhile, persistent downward motion suppressed DGPI over the BoB, although its suppressive effect was partly offset by reduced vertical wind shear and gradual SST warming, thereby sustaining TCG over the BoB. Interdecadal shifts in the Indian Ocean Dipole (IOD) may contribute to the contrasting TCG changes between AS and BoB from P1 to P2. The negative IOD (nIOD) events are more prevalent during P1, whereas the relative frequency of positive IOD (pIOD) events increases markedly in P2. Correspondingly, P2 exhibits a more pronounced pIOD‐like warming pattern, characterised by stronger warming over the AS than over the BoB. Under this altered mean state, pIOD‐related signals exert enhanced influences on regional atmospheric conditions during P2, while the regional impacts associated with nIOD events may also become amplified. As a result, IOD‐related SST and circulation anomalies exhibit stronger correspondence with the atmospheric conditions favourable for TCG identified by DGPI over the AS and BoB during P2. This enhanced coupling strengthens the coherent modulation of TCG between the AS and BoB during P2, potentially contributing to the correlation shift between the two basins.