The Cause of an Extremely Low Salinity Anomaly in the Bay of Bengal During 2012 Spring

An extremely low salinity anomaly was observed in the Bay of Bengal (BoB) by various in situ and satellite observations during 2012 spring and ranked as the strongest anomaly in the last two decades. By analyzing various observational and reanalysis datasets, we find that the reversing circulation anomalies caused by a rare co‐occurrence of positive Indian Ocean Dipole (pIOD) and La Niña in 2011 were the main cause of this extreme event. In the fall of 2011, the pIOD favored an anticyclonic circulation anomaly flowing from the BoB into the equatorial Indian Ocean, whereas the La Niña event, which matured in the following winter, sharply reversed the circulation anomaly to a cyclonic pattern. As a result, these reversing circulation anomalies inhibited the southward freshwater transport along the two sides of the BoB and trapped substantial freshwater to the northern bay by the end of winter. Thereafter, the strong seasonal anticyclonic circulation transported this freshwater into the central and southern bay from the northeastern BoB coastal area, forming the extremely low salinity anomaly during 2012 spring. The key process in forming the extremely low salinity anomaly was the modulation of the seasonal monsoon by interannual processes, specifically the rare combination of pIOD and La Niña in 2011. Our results highlight the important role of the oceanic circulation in the sea surface salinity (SSS) variability in the BoB. This study provides some new perspectives on the SSS interannual variability in the BoB and the freshwater exchange between the BoB and Arabian Sea.