Impact of SST on the intensity prediction of Extremely Severe Tropical Cyclones Fani and Amphan in the Bay of Bengal

Abstract

This study investigates the impact of SST on the prediction of intensification / rapid intensification (RI) of two pre-monsoon Tropical Cyclones Fani (2019) and Amphan (2020) in the Bay of Bengal using convection-permitting high-resolution WRF simulations. Sensitivity experiments are conducted with SST boundary conditions derived from i) NCEP operational GFS SST analysis and forecasts (GFS), ii) NOAA real-time SST data (NOAA) iii), Climatological mean SST (CLIM). Simulations indicated a positive SST anomaly of 1.5 °C, 0.75 °C for Amphan and Fani respectively in NOAA and GFS and all simulations revealed moderate wind shear during intensification. Significant differences are found in the intensity prediction. NOAA followed by GFS predict relatively stronger TCs in response to the warm SST anomalies relative to CLIM. Results of NOAA suggest that it creates a more thermodynamically favourable environment than GFS and CLIM due to warm SST which would facilitate an intense tilt-induced asymmetric convection, stronger upper air divergence, and its interaction with the environment flow tending to reduce the shear and leading to the RI. A detailed analysis for Amphan revealed an increase of surface fluxes, stronger diabatic heating, enhanced low-level convergence and convection indicating the creation of a more favourable thermodynamic environment in NOAA compared to GFS and CLIM due to warm SST, which favors intensification/RI. The positive SST anomaly present over a large area around the cyclone in NOAA has led to larger surface enthalpy fluxes, a high θe in the lower and upper regions as well as a warmer core with strong convective updrafts all leading to a higher intensification relative to GFS and CLIM.