Seasonal and Interannual Variability of Sea Surface Salinity in Bay of Bengal

Heavy rainfall and strong river runoffs associated with the southwest monsoon makes Bay of Bengal (BoB) one of the freshest regions in the tropical oceans. This surface freshening results in strong near surface salinity stratification, which has strong impacts on the atmosphere and regional climate. Due to the paucity of in-situ sea surface salinity (SSS) observations and the lack of reliable interannually varying freshwater forcing, little is known about the mechanisms governing the seasonal and interannual SSS evolution in the bay. We therefore use an ocean general circulation model without relaxation towards observation and forced with interannual estimates of river runoffs and precipitations to study the SSS variations. The model accurately simulates the SSS fluctuations of the in-situ data. The largest seasonal and interannual SSS fluctuations occur at the head of the Bay and along the eastern coast of India. The strong seasonal freshening that occurs shortly after the monsoon in the northern part of the Bay and its interannual fluctuations are both driven by the variability of Ganges-Brahmaputra river discharge. Horizontal advection by the southward flowing coastal current is responsible for the seasonal southward expansion of fresh pool along the east coast of India. The interannual SSS fluctuations in this region are remotely controlled by the Indian Ocean Dipole. Contrary to what was thought before, the salt influx into the upper BoB occurs primarily through turbulent vertical exchanges with the underlying saltier waters, rather than by horizontal exchanges with the rest of the Indian Ocean.