Variability of Sea Surface Salinity in the Southeastern Arabian Sea: Driving mechanisms and influence on the Arabian Sea mini Warm Pool

With sea surface temperatures (SST) exceeding 30˚C in May, the southeastern Arabian Sea (SEAS) hosts one of the warmest open ocean region globally, which appears to play an important role in the summer monsoon onset. Freshwater input from the Bay of Bengal precede the SEAS warm pool build-up by a few months, and are believed to influence its temperature through its impact on oceanic stability and vertical mixing of heat. SSS interannual variations in the SEAS region have not been extensively described before, and their potential feedback on the warm pool build-up and the monsoon are still debated. In the present study, we describe the SEAS SSS seasonal and interannual variability, its driving mechanisms and potential impact on the monsoon. To that end, we analyse experiments performed with a regional 25-km ocean model, both forced and coupled to a regional atmospheric model. The forced and coupled simulations both reproduce the main oceanic features in the SEAS region, including the salinity seasonal cycle and interannual variability. Winter salinity stratification inhibits the vertical mixing of heat, thereby warming the mixed layer by ~0.5°C.month^-1. This salinity-induced warming is however compensated by a salinity-induced cooling by air-sea fluxes. Salinity stratification indeed yields a thinner mixed layer which is more efficiently cooled by negative surface heat fluxes at this season. Overall, salinity has thus a negligible impact on the SST seasonal cycle. SEAS SSS interannual variations are largely remotely driven by the Indian Ocean Dipole (IOD), an indigenous interannual climate mode in the equatorial Indian Ocean. The IOD remotely impacts coastal currents along the Indian coastline, and hence modulates freshwater transport from the Bay of Bengal into the SEAS. This yields positive SSS anomalies in the SEAS during the boreal winter that follows positive IOD events. Those SSS anomalies however do not appear to significantly alter the interannual surface layer heat budget. Coupled model sensitivity experiments, in which the influence of haline stratification on vertical mixing is neglected, further confirm that the SEAS winter freshening does not significantly influence the SEAS warm-pool build-up nor the monsoon onset