RESPECTIVE ROLES OF AIR-SEA COUPLING AND ATMOSPHERIC BACKGROUND CONDITIONS ON SEASONAL CHANGES OF TROPICAL CYCLONE CHARACTERISTICS BETWEEN THE PRE AND POST-MONSOON IN THE BAY OF BENGAL

The Bay of Bengal (BoB) is one of the most vulnerable regions to Tropical Cyclones (TCs) globally, which poses a serious threat to coastal countries along its rim. BoB TCs occur during two favourable periods, just before and after the Indian monsoon. Previous studies have demonstrated that the strong post-monsoon salinity stratification inhibits vertical mixing and reduces the sea surface cooling under TCs. In the present study, we investigate whether this reduced cooling favours the development of stronger TCs during the post- than during the pre-monsoon. We use a ¼° regional ocean-atmosphere model of the Indian Ocean, which simulates TCs and related air-sea interactions reasonably over the BoB. Contrary to previous suggestions, the model and observations both display stronger TCs during the pre-monsoon. This is because seasonal changes in atmospheric environmental parameters dominate those associated with air-sea coupling. The pre-monsoon indeed exhibits weaker vertical wind shear, increased relative humidity and reduced stability of the atmosphere column, which all favour the development of stronger TCs. A sensitivity experiment that does not account for air-sea coupling under TCs reveals that air-sea coupling considerably reduces the number of TCs during both seasons. Air-sea coupling also results in a substantial reduction of the TCs intensity during the pre-monsoon, because of larger TCs-related surface cooling during this period. Effects of air-sea coupling thus alleviate the seasonal contrasts in TCs intensity that result from seasonal changes in background atmospheric state.