Impact of the Upper Ocean Processes on Intensification of Cyclone Amphan

Abstract

Cyclone Amphan is one of the severe cyclones that occurred over the Bay of Bengal region. It has undergone rapid intensification over the northern part, i.e., north of 14� N after its formation. This can be attributed to the intrusion of warm saline waters with temperatures around 31 �C, salinity greater than 33 psu from the coast, and the presence of anticyclonic eddy that moved across the track. Within an anticyclonic eddy sinking, warm, high saline waters are promoted further sinking through diffusive convective mixing processes. Analysis of in situ and model data showed that these waters formed as a thick water column with high heat content in the northern part of the track, which in turn helped the rapid intensification of cyclone Amphan. This thick water column was also clearly evident in tropical cyclone heat potential. A high chlorophyll concentration of 4.5 - 5.3 mg/m3 was observed before the saline water intrusion across the cyclone path. North of 15oN, a sudden drop in the surface chlorophyll concentration of 0.2-1 mg/m3was observed, which can be attributed to further sinking of saltwater due to vertical convective mixing, which suppressed the cyclone-induced upwelling over this region.