Dynamic and thermodynamic upper-ocean response to the passage of Bay of Bengal cyclones 'Phailin' and 'Hudhud': a study using a coupled modelling system

Understanding the upper-ocean response to tropical cyclones (TCs) in terms of sea surface temperature (SST) cooling is of prime importance in the prediction of TC intensity. However, the magnitude of cooling during the passage of TC varies depending on storm characteristics and pre-existing upper-ocean conditions such as the presence of ocean eddy and upper-ocean stratification. The present study investigates the upper-ocean response to two post-monsoon Bay of Bengal (BoB) cyclones, Phailin (October 2013) and Hudhud (October 2014), those followed almost a similar track, in association with pre-existing oceanic conditions using a fully coupled ocean-atmosphere modelling system. The spatial structure and temporal evolution of SST cooling induced by the two cyclones and the physical processes governing the cooling are examined. Analysis shows that the intensity of Phailin is significantly reduced when it encountered the regime of lower tropical cyclone heat potential (TCHP) associated with pre-existing cold core eddy (CCE). Intense upwelling with an average of 0.6 m/h is observed over CCE that resulted in strong temperature tendency of - 4.2 degrees C prior to landfall. Though average TCHP in the generation region of Hudhud was 50 kJ/cm(2), the storm drew sufficient energy from the underlying ocean due to its slow translation speed. Presence of shallow thermocline over extended region and weaker upper-ocean stratification enhanced SST cooling over a larger region after passage of the TC Hudhud. Finally, the present study brings in clarity that the upper-ocean condition and the relative position of the mesoscale oceanic features to the storm track are responsible for the intensification of the TC and the recovery of the ocean surface.