Oceanic and Atmospheric factors contributing towards the rapid intensification of tropical cyclones in a warming climate: A diagnostic study of Super Cyclone AMPHAN over the Bay of Bengal

The super cyclone AMPHAN, which devastated the lives of millions, was developed over the Bay of Bengal (BoB) in late May 2020. Whether the air-sea interaction of tropical cyclones over the Bay of Bengal has changed or will change in a warming climate demands further attention because of its ecological importance, too. In the context of climate change, it is crucial that we understand the potential modifications in the air-sea interaction during the life cycle of the super cyclonic storms, observed through previous studies. We address this issue through a case study of cyclone AMPHAN. The present analysis focuses on oceanic and atmospheric conditions before and during the super cyclonic storm AMPHAN. Data analysis reveals that abnormal warming in ocean temperatures was observed prior to its genesis, compared to past tropical cyclones, which led to the rapid intensification of AMPHAN from category 1 to category 5 within a short time span of 18 hours. In this study, we also explore the physical mechanisms that drove this rapid intensification and associated dynamic and thermodynamic features using hourly data from ERA5 reanalysis. We also used the satellite and RADAR observations during the super cyclone. The three-dimensional structure of diabatic heating, which represents the pathway leading to the quick intensification of AMPHAN, was also studied. The diabatic heating is estimated as a residual in the thermodynamic equation using the full resolution 37-level ERA-Interim 6 hourly reanalysis. The diabatic heating is associated with radiative fluxes, phase changes of water substance, and turbulent flux of sensible heat from the earth's surface, thus it includes precipitation. Precipitation characteristics were also examined during the life cycle of this super cyclone. High mid-tropospheric humidity and weak vertical shear of horizontal winds were observed, which is unusual in May as per climatology. As a result, the vertical velocity increased and triggered the convection and subsequent rapid intensification of the super cyclonic storm. Favorable subsurface ocean conditions are also responsible for such rapid intensification.