Influence of river discharge and tides on the summertime discontinuity of Western Boundary Current in the Bay of Bengal

Abstract

East India Coastal Current (EICC), the Western Boundary Current (WBC) in the Bay of Bengal (BOB), is continuous and well–directed during pre– and post–Monsoon, but discontinuous during monsoon (June–September). This study examines the individual and combined effects of river discharge and tidal forcing on the EICC discontinuity using high resolution (1/12°) Regional Ocean Modeling System simulations. Four climatological experiments, a control simulation with normal boundary conditions and three other sensitivity simulations with the same boundary conditions, but with river input, tidal forcing, and both together are conducted. The analysis shows that, during ISM, the southward reversal of EICC from head bay is enhanced with the river input, while the tide forcing strengthens the northward EICC from north of Sri Lanka. High horizontal salinity gradient flow in the stratified upper ocean caused by the river discharge increases the surface currents. High vertical mixing in tide forcing suppresses the surface features. The strong horizontal diffusivity due to river discharge promotes the eddy genesis and propagation throughout the western BOB. Conversely, tidal oscillation contributes high turbulent buoyancy, which makes the upper ocean relatively unstable, and the discontinuity remains confined to the western boundary. The combined forcing simulation indicates the dominance of river discharge in the upper layers with suppressed surface features due to tides, which intensify the discontinuity at subsurface. Therefore, the results of this numerical study suggest that the river input and tidal forcing both play important and complimentary roles in maintaining the realistic summertime discontinuity in the BOB.