What Made the Sustained Intensification of Tropical Cyclone Fani in the Bay of Bengal? An Investigation Using Coupled Atmosphere–Ocean Model

The extremely severe tropical cyclone Fani (25 April–5 May 2019) unusually sustained high intensity for a prolonged duration over the Bay of Bengal (BoB). A regional coupled atmosphere–ocean model was used to investigate the atmospheric and oceanic conditions and processes responsible for the sustained intensification of the tropical cyclone (TC) Fani. The coupled model simulated the track and intensification/weakening stages of the cyclone reasonably well. A reduction in sea surface temperature (by −2 °C) and an increase in sea surface salinity due to cyclone-induced upwelling and inertial mixing was noticed in both observations and model. The passage of TC Fani over two geostrophic mesoscale warm-core eddies along the cyclone track was found to supply the necessary energy for the intensification of TC Fani. The sea surface height anomaly and tropical cyclone heat potential (TCHP) were higher during TC Fani than other pre-monsoon cyclones in the BoB. The anomalous TCHP in the warm-core eddy zones (i.e., in excess of >160 kJ cm−2) maintained the warm surface temperature and high air–sea heat fluxes. The air–sea latent heat flux and atmospheric wind shear were favourable for the intensification of the cyclone. The atmospheric moist static energy enhanced up to 360 kJ kg−1 with a deep vertical extension in the atmospheric column supporting the further intensification of TC Fani. Therefore, the unusual oceanic TCHP associated with mesoscale eddies, higher latent heat flux, and enhanced moist static energy in the atmosphere contributed to the sustained intensification of TC Fani for a prolonged period in the BoB.