Impact of Tropical Cyclones on Physical and Biological Response of the Bay of Bengal Using Observations and Model Simulations

The air-sea interaction processes during the Tropical Cyclone (TC) formation and intensification affect the upper ocean mixing processes thereby changing stratification parameters and vigour of upwelling which ultimately enhances biological activity. Hence the current study conducts a comparative analysis of surface chlorophyll (Chl-a) changes during four tropical cyclones viz. Mala (pre-monsoon) cyclone, Nilam cyclone, Phailin cyclone, and Hudhud (post-monsoon) cyclone occurred in the Bay of Bengal (BoB) using numerical modelling. The study depicts the influence of net heat flux and winds on the variability of Chl-a concentration, Mixed Layer Depth (MLD), and Sea Surface Temperature (SST) along the track of the cyclones. Here, we have performed three experiments using Regional Ocean Modelling System (ROMS) coupled with a fourcomponent ecosystem model with daily and climatological forcing for Wind Stress Curl (WSC) and Net Heat Flux (NHF). The first experiment (viz. the control run) includes the daily forcing of WSC and NHF during cyclone days. The second experiment comprises only the daily WSC with climatological NHF while the third experiment considers the daily forcing of NHF and climatological WSC. Stratification parameter is estimated for all the three experiments to find out which experiment is well suited for maximum mixing during and post-cyclone periods. The results showed that during Mala cyclone the winds are more effective and play an important role in mixing. The reason for the weaker impact of NHF may be attributed to higher heating over the ocean surface in summer which tends to increase the stratification. The reason may be attributed to NHF, as the flux during post-monsoon season is negative which suggest that ocean surface is warmer which is indicative of a more congenial environment for mixing. On the other hand, it is analysed that the NHF and the WSC play an important role in mixing for the Nilam, Phailin, and the Hudhud cyclones. This implies that in addition to strong WSC, higher heat loss from the ocean surface during winter increases the density of surface waters and the water column becomes convectively unstable leading to vertical mixing and changing the surface Chl-a concentration.