The Role of Temperature Inversions in the Generation of Seasonal and Interannual SST Variability in the Far Northern Bay of Bengal

Abstract

he northern Bay of Bengal is characterized by freshwater supply from the Ganges and Brahmaputra Rivers. The resulting shallow haline stratification and thick barrier layer lead to temperature inversions in fall and winter, that is, cool surface water overlaying warm subsurface water. This study examines sea surface temperature (SST) variability off Bangladesh and shows that temperature inversions play an essential role in generating seasonal and interannual SST variability there. Two satellite SST datasets reveal that the magnitude of SST variability has a local peak near the coast of Bangladesh on seasonal and interannual time scales. Output from a high-resolution ocean general circulation model, which is validated by satellite SST and Argo float observations, is used to calculate the mixed layer heat budget. Results show that inverted temperature profiles lead to SST warming on the seasonal time scale via heat exchange at the bottom of the mixed layer, which balances climatological atmospheric cooling in fall and winter. On interannual time scales, surface heat flux tends to damp SST variability, whereas heat exchange at the base of the mixed layer contributes to the growth of SST anomalies. SST off Bangladesh tends to be anomalously high in the year after an El Niño event and in the year of negative Indian Ocean dipole and La Niña events. The atmospheric circulations related to these climate modes force anomalous Ekman pumping, which advects more subsurface warm water to the surface in fall and winter, resulting in anomalous mixed layer warming. The deepening of the mixed layer entrains more subsurface warm water, which also contributes to anomalous warming.