Relative contribution of eddies ant atmospheric forcing to the Bay of Bengal non-seasonal Sea Surface Salinity Variability

The Bay of Bengal (BoB) Sea Surface Salinity (SSS) is highly contrasted and variable, in response to the large monsoonal wind and freshwater forcing. In addition to this strong seasonal cycle, previous studies have underlined strong SSS non-seasonal variations associated with the Indian Ocean Dipole (IOD) and mesoscale eddies. In this study, we quantify the relative contributions of externally forced (wind, freshwater) and internally generated (mesoscale eddies) SSS non-seasonal variability in the BoB. To that end, we use Ocean General Circulation Model 10-member ensemble experiments from the IMHOTEP (IMpacts of freshwater discHarge interannual variability on Ocean heaT-salt contents and rEgional sea-level change over the altimetry Period) project.
The model reproduces the large forced interannual SSS signals in the Northernmost part of the BoB and along the east coast of India, associated with the East Indian Coastal Current (EICC) modulation by the IOD. The internal SSS variability is largest in boreal fall in the North-Western BoB and more tightly controlled by the climatological SSS gradient distribution than by that of eddy kinetic energy. The external atmospheric forcing dominates the total variability in the regions of strongest variability, near the Ganges mouth and along the east coast of India in boreal fall and winter. Internal variability, however, contributes to 50-70% of the variability further offshore in boreal fall and winter. This confirms the strong role of eddies in controlling the freshwater extension up to ~700 km away from the coast, through stirring of the intense gradient between the coastal fresh and offshore saltier water. We finally discuss the consequences of these findings for comparing model and observations, in view of the chaotic nature of internal eddy variability.