Simulation of the upper oceanic response to the super cyclonic storm Amphan in the Northern Bay of Bengal

Abstract

The lack of observational data in the northern part of the Bay of Bengal (BoB) makes it challenging to investigate the upper oceanic responses to tropical cyclones. To overcome this challenge, a high-resolution Regional Ocean Model (ROMS) is set up with horizontal resolution of 0.03° × 0.03° and 50 vertical layers for the northern BoB, with the daily output of the First Institute of Oceanography surface wave-tide-circulation coupled ocean model (FIO-COM) as the boundary condition, and hourly ERA5 data as atmospheric forcings. This regional model is systematically validated and then utilized to reconstruct the upper ocean response due to the super cyclonic storm Amphan over the BoB from May 16 to 20, 2020. The upper ocean responses to Amphan in the northern BoB is well reconstructed with this regional model. On the right side of the cyclone track, sea surface temperature (SST) cooling (4 °C) and increased sea surface salinity (0.5 psu) are well reproduced. The primary oceanic triggering forces intensified the cyclone through the extraordinarily high SST (>31 °C) and deep isothermal layer depth. Tropical cyclone heat potential was high (over 100Kj cm⁻²) during the early stages of the cyclone, which aided the transformation of a depression into a super cyclonic storm. Vertical entrainment and horizontal advection had a major influence in the pronounced cooling within the mixed layer.