Contrasting the upper ocean response to two intense cyclones in the Bay of Bengal

Abstract The contrasting upper ocean responses of the Bay of Bengal to two intense cyclones, Phailin and Hudhud, which traversed similar paths and occurred at nearly the identical time of year, were analysed using observations from moored buoys, satellite observations, and modelling. Our analyses revealed that the upper ocean was highly stratified prior to Phailin compared to the case for Hudhud. The stratification in the near surface prior to the onset of Phailin (Hudhud) was predominantly driven by vertical gradients in salinity (temperature). The upper ocean response to the passage of Phailin (Hudhud) was characterised by notable changes in sea surface salinity (sea surface temperature). In addition, enhancement of chlorophyll-a was observed over an extensive area following the passage of Hudhud but was confined to a mesoscale eddy after Phailin. The presence of strong near-surface haline stratification sustained temperature inversion and resulted in weak sea surface temperature cooling in the wake of Phailin. The mixing centred along the trajectory of a cyclone in a haline (thermally)-stratified ocean leads to fronts having strong horizontal haline (thermal) gradients in the wake. Cross-front ageostrophic submesoscale circulation features resulted in rapid fluctuations of salinity in the wake of Phailin and temperature in the wake of Hudhud. The cross-front ageostrophic circulation features and post-storm winds played a critical role in controlling mixed layer restratification after the cyclones.