Is a warming northern Indian Ocean generating more tropical cyclones?

Tropical cyclones in the northern Indian Ocean (IO) were examined in the context of basin-specific warming from 1960 to 2022. In the Arabian Sea (AS) the increase in the total number of cyclones on a decadal-time scale closely followed basin-averaged rapid warming and reflected the presence of environmental conditions congenial for cyclogenesis. The increased occurrence of higher category cyclones in the AS after 1995 was closely linked with the accelerated warming and a 5-fold increase in the tropical cyclone heat potential (TCHP). Changes in the dynamic variables such as decreased vertical wind shear and increased relative vorticity, and increase in the thermodynamic variables viz. TCHP and relative humidity acted in tandem to create conditions favourable for generation of a greater number and more intense tropical cyclones after 1995. This was facilitated by the increased occurrence of positive Indian Ocean dipole (IOD) after 1995 which deepened the isothermal layer and modified the environmental parameters through altered Walker circulation in the AS. In contrast, increased occurrence of positive IOD cooled the eastern IO and that resulted in the observed reduced rate of warming in the Bay of Bengal (BoB) after 1995. The lack of trend in the total number of cyclonic systems on decadal-time scale in the BoB was linked to non-supportive dynamic environmental conditions for cyclogenesis. The continued occurrence of higher category cyclones in the BoB, despite the lack of decadal trend, could be explained, in part, by the higher magnitudes of tropical cyclone heat potential and higher atmospheric moisture content.