Why Does the Bay of Bengal have a Strong Shallow Stratification?

Strong stratification in the upper 20m of the Bay of Bengal during the southwest monsoon limits the depth of mixing, thereby shortening the response time of ocean SST to intraseasonal oscillations and allowing the ocean to play a role in their dynamics. Although freshwater input from rain and from the Ganges-Brahmaputra and Irrawaddy river systems provides the necessary lateral density contrast, the mechanisms by which this is converted to a vertical stratification have remained unclear. One possible mechanism is submesoscale instabilities and eddies that slump horizontal gradients into the vertical at fronts, driven by the available potential energy in the gradient. Another is the vertical shear in the Ekman layer that advects lighter fresher water over denser water, driven by the wind. These two mechanisms have distinctly different physics, time and space scales and driving mechanisms. Here, we examine ship-based and autonomous data from 2014 and 2015 in the Bay of Bengal, taken as part of the ASIRI/OMM project joint between US and Indian scientists for evidence for the importance of these two mechanisms.