Seasonal variability of sea surface salinity and salt budget of the mixed layer of the north Indian Ocean

A subset of the recently published salinity database of the global oceans is utilized to characterize and explain the observed seasonal variability of sea surface salinity of the north Indian Ocean, in greater detail than has been possible previously. The influence of salinity on the seasonal evolution of near-surface mixed layer depth is highlighted. The relative importance of freshwater flux (evaporation minus precipitation) and horizontal advection in accounting the observed seasonal variability of sea surface salinity is evaluated. The influence of massive river outflow in producing the observed sea surface salinity minima in the coastal northwestern Bay of Bengal during August-September is highlighted. The observed interannual variability of sea surface salinity along two major shipping lanes in the tropical Indian Ocean in relation to El Nino is examined. The annual average of sea surface salinity shows contrasting distributions in the Arabian Sea and the Bay of Bengal due to differences in hydrological forcing. The seasonal variability of sea surface salinity is most pronounced in the coastal region of the northern Bay of Bengal, northwestern Arabian Sea, and the southeastern Arabian Sea. The incorporation of salinity effect reduces the thickness of the near-surface mixed layer, and this reduction is most pronounced in the Bay of Bengal, where it builds up from June to July and becomes most prominent by February in the following year, when the freshening effects of hydrological forcing through local rainfall and river discharges are felt the most on the near-surface layers. The salt budget analysis of the mixed layer shows a broad agreement between the patterns of observed and diagnosed seasonal changes caused by freshwater flux and horizontal advection, despite limitations in the accuracy of these estimates. The freshwater input through rainfall and river discharges in the Bay of Bengal far exceeds evaporation, causing surplus freshwater for export. Horizontal advection of salinity is found to be important in the southeastern Arabian Sea during winter and in the western and eastern Arabian Sea during the summer monsoon season and in the Bay of Bengal throughout the year with the exception of premonsoon season. The pronounced dilution observed during the height of the summer monsoon season in the coastal northwestern Bay of Bengal is attributed to peak discharges from major rivers. Historic data along two major shipping lanes in the tropical Indian Ocean have clearly revealed the signature of El Nino in the interannual variability of sea surface salinity.