Response of a high-resolution ocean circulation model to winds from different sources in simulating summer monsoon freshening in the North Bay of Bengal: A case study

A very high-resolution model for the Bay of Bengal is configured to simulate submesoscale flow features, which are otherwise absent in coarse resolution models. In this manuscript, we present the response of this model to simulate summer monsoon freshening in the northern Bay of Bengal when forced with two different winds, one based on weather forecast model output from National Centre for medium range weather forecast (NCMRWF) and the other based on satellite-derived SCATSAT ocean surface winds for 2017 as a case study. Our results reveal that the spatio-temporal evolution of lateral freshwater dispersal in the northern Bay of Bengal is sensitive to wind vorticity and eddy formation/evolution. A reasonably realistic decrease of 2–3 psu in the sea surface salinity at the mooring location of 18oN, 90oE towards the end of September 2017 is simulated in the model when forced with the SCATSAT winds, whereas significantly weaker freshening occurs at the mooring location when the model is forced with the NCMRWF winds. A filament of strong positive vorticity connecting two cyclonic eddies in the north central Bay in the NCMRWF wind forced simulations prevents the direct southward advection of freshwater, resulting in higher salinity at the mooring location. A similar mechanism that would prevent southward advection of fresh water is absent in the SCATSAT forced model run. Westward moving Rossby waves in the SCATSAT forced model run are found to play an important role in the evolution of freshening events at various locations in the northern Bay of Bengal. Lagrangian Coherent Structures (LCS) derived from the model currents suggest that the background conditions were favourable to advect freshwater from Ganges-Brahmaputra towards the mooring location in the northern Bay of Bengal in the SCATSAT forced simulations.