Increased mixed layer salinity during the monsoon intraseasonal Oscillation in the Northern Bay of Bengal

Abstract The northern Bay of Bengal (BoB) exhibits low mixed layer salinity accompanied by strong intraseasonal variability, primarily driven by the Indian Summer Monsoon and monsoon intraseasonal oscillation (MISO). Notably, the northern BoB shows concurrent increases in mixed layer salinity and MISO-induced precipitation at intraseasonal timescales. This contrasts with the central BoB where precipitation typically reduces salinity in mixed layer. Diagnostic studies reveal that horizontal advection, rather than freshwater flux, is the dominant term in this process particularly to the north of 18°N during the MISO. Wind-driven meridional currents transport saline water northward, increasing salinity before MISO arrival; subsequently, reversed winds reduce salinity as MISO propagates northward through changing horizontal advection. In the latitudinal band of 15°N–18°N, horizontal advection consistently decreases salinity, but MISO-induced southwesterly winds enhance evaporation, yielding net salinity increases. These results elucidate the mechanisms governing mixed layer salinity variations in the BoB, underscoring the complexity of air-sea interactions during MISOs.