Wind forcing of the Ganga-Brahmaputra river plume

Response of Ganga-Brahmaputra river plume to wind forcing in the Bay of Bengal is studied using a numerical ocean circulation model. Four different wind forcing scenarios, namely, winds over the entire model domain, no winds anywhere over the model domain, winds over Equatorial Indian Ocean only and winds over Bay of Bengal only, are considered. Model simulations are carried out in an idealized setting where forcing from other rivers and precipitation is ignored. Despite the absence of this forcing, model captures observed phases of Ganga-Brahmaputra river plume, seasonal cycle of sea surface temperature and spatio-temporal structure of East India Coastal Current (EICC) reasonably well. Horizontal structure of the plume is investigated using surface salinity, surface currents and freshwater thickness obtained from the simulation that includes Ganga-Brahmaputra river discharge and winds over the entire model domain. The plume spreads upstream (eastward) but remains confined to the coast in northern bay during spring. During summer monsoon, plume spreads southward along the east coast of India and subsequently southeastward over the open bay and reaches the northern tip of Andaman islands by the end of October. During winter monsoon, the plume flows southward, assisted by EICC along the east coast of India and recedes northward in the central bay. In the absence of winds, the plume flows southward along the coast of India throughout the year. Equatorial winds force the plume farther (compared to no winds case) southward along the coast of India prominently in the winter monsoon. Local winds control the horizontal structure of the plume in the bay as they produce the seasonal structure that closely resembles that produced by the winds over the entire model domain. Momentum balance of the plume reveals that geostrophy controls the westward drift of the freshwater around the river mouth. Wind friction and associated Ekman flow are important in the upstream spreading of the plume during spring and in the eastward expansion over the open bay during summer monsoon.