Vertical mixing of high salinity and low salinity in the Bay of Bengal: Results from a coupled high-resolution atmosphere-ocean-wave model.

The Indian Peninsula is a barrier between two distinct water masses in the northern Indian Ocean: The high salinity waters in the Arabian Sea associated with the regional desert climate are separated from the low salinity waters in the moist tropical Bay of Bengal. The two water masses meet in the equatorial band south of Sri Lanka and the boundary between them moves west to east and vice versa as the annual cycle of the Asian monsoon changes from southwest monsoon to northeast monsoon. Vertical mixing in the interior of the Bay of Bengal is required to explain the observed salinity in water masses that leaves the bay.

In this work we use a very high resolution fully coupled atmosphere-ocean-wave model, the US Navy’s Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS). It is applied to the Bay of Bengal region has been run during both Northeast Monsoon (NE) (Dec 2014) and Southwest Monsoon (SW) conditions as part of the Office of Naval Research ASIRI (Air-Sea Interactions in the Northern Indian Ocean) project. Since May 1, 2015, the model has been updated daily to show the 10-m wind speed, currents, sea surface height, sea surface temperature, surface salinity, net surface heat flux into the ocean and significant height and direction of surface waves on a web-site available to ASIRI researchers. We find intrusions of high-salinity water Arabian Sea, that spreads out along isopycnals deeper than 75 m into the Bay of Bengal. The spiciness along isopycnals in the bay shows strong gradients in temperature and salinity in eddies, which indicates that mixing of low salinity water and high salinity water can occur in those regions. We also find that semi-diurnal internal waves generated by tides contribute to the mixing in the Bay of Bengal. We will show model results and observations that emphasize the role of mixing, and the regions where mixing between low and high salinity waters appears to take place.