Role of Ocean Sub-surface Processes in the Intensification of Tropical Cyclone Phailin: A Coupled Model Study

The ocean plays an important role in the intensification of tropical cyclones (TC), and yet there is an immense gap in understanding the ocean sub-surface processes responsible for it. There also appear to be a very limited number of studies in this topic, especially in the context of Bay of Bengal (BoB). The present study is aimed to investigate the ocean sub-surface processes and their contributions in the intensification of TC Phailin (2013) over BoB, using the Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) model. Three sub-surface ocean conditions, viz., a) European Centre for Medium Range Weather Forecast (ECMWF) Ocean Reanalysis, b) Climate Forecast System Version 2 (CFSV2) Operational Analysis, and c) HYbrid Coordinate Ocean Model (HYCOM) Reanalysis, are used as the initial and boundary conditions for the oceanic component of the coupled model. All the simulations showed a delay in intensification and among the three experiments, ECMWF simulated the most intensified TC. CFSV2 simulated a deeper mixed layer and higher mixing which hindered the intensification of the TC in this experiment. Furthermore, higher entrainment of cold water in the mixed layer (ML) subsequently resulted in cold water reaching the surface, leading to a consequent decrease in the sea surface temperature (SST), which acted as a negative feedback in the intensification of the TC in CFSV2 and HYCOM. ECMWF realistically captured the enhanced upwelling and the interactions with a cold-core eddy before landfall. Sudden increase in the ML depth leading to the increase in the ML heat content, addition of heat in the ML due to entrainment, and the prevention of cold water reaching the surface, as seen in ECMWF, are indicative of breaking of the barrier layer. This acted as a positive feedback mechanism in the intensification of the TC in ECMWF, enabling the model to capture the intensity with highest accuracy in this experiment. The findings of this study strongly suggest that not only the accurate incorporation of oceanic sub-surface physical processes is important, but also proper representation of the distribution of temperature and salinity profiles and prevailing mesoscale features are essential for realistic coupled model simulations of TCs.