Observed interannual variability of near‐surface salinity in the B ay of B engal

Abstract An in situ gridded data of salinity, comprising Argo and CTD profiles, has been used to study the interannual variability of near‐surface salinity (within 30 m from sea surface) in the Bay of Bengal (BoB) during the years 2005–2013. In addition to the broad agreement with earlier studies on the north‐to‐south gradient of surface salinity and general features of seasonal variability of salinity, the data also revealed few episodes of enhanced freshening in the BoB. The observations showed distinct anomalous low salinity (< 2 psu) waters in the northern BoB during June–February of the years 2006–2007 (Y67), 2011–2012 (Y12), and 2012–2013 (Y23). The anomalous freshening during these years showed similar life cycle, such as, it starts in the northern BoB during July–September of current summer and extends up to February–March of next winter with a southward propagation. Analysis showed that the oceanic and atmospheric conditions associated with positive Indian Ocean Dipole (pIOD) lead to these freshening events, and IOD rather than El Niño/Southern Oscillation (ENSO) controls the interannual variability of salinity in the BoB. The mixed layer salt budget analysis indicated the dominant role of local fresh water flux (horizontal advection) on the observed salinity tendency during summer (winter) monsoon season. Enhanced precipitation associated with pIOD lead to enhanced freshening in northern BoB during June–September, which remained to this region with prevailing summer monsoon circulation. The weakening or absence of southward east India coastal current (EICC) during October–December of these freshening years trapped anomalous freshwater in the northern BoB.