Role and influence of key atmospheric parameters in large-scale environmental flow associated with tropical cyclogenesis and ENSO in the North Indian Ocean basin

Intense cyclones have become more frequent over the North Indian Ocean (NIO) region causing significant risk and vulnerability to the coastal regions. Analysis clearly revealed an increasing trend (similar to 16.75%) in the intensity of severe cyclonic storms that formed over the NIO during the past 4 decades. Power dissipation index (PDI) showed a correlation of 0.58 with severe cyclonic storms during the study period (1979-2019). Study reveals that the genesis location for post-monsoon cyclones is statistically significant against the genesis potential index (GPI) difference, especially for the intense cyclone categories (>48 kt). During the La Nina years, increased tropospheric water vapor content initiates over prominent locations in the Andaman Sea and the North China Sea. Also, La Nina periods showed positional shift in the average genesis location of intense tropical cyclones compared to El Nino periods. Cyclogenesis shift is more concentrated towards the west-equatorward directions over the Bay of Bengal basin. Strong mid-level relative humidity (RH600), positive low-level relative vorticity (RV850), weak vertical wind shear (VWS), warm sea surface temperature (SST), and suppressed outgoing long wave radiation (OLR500) are the responsible factors leading to increased tropical cyclone activity in the NIO. Study signifies that RH600, RV850, and OLR500 are distinct during pre-monsoon seasons of the La Nina epoch, favoring the genesis of severe cyclones over this region. Variables such as SST, wind streamlines (W-200, W-850), vertical velocity (VV500), and specific humidity (SH500) exhibit comparable contributions towards cyclogenesis during both El Nino and La Nina phases.