Subseasonal Dispersal of Freshwater in the Northern Bay of Bengal in the 2013 Summer Monsoon Season

Abstract

AbstractNear‐surface salinity in the north Bay of Bengal (BoB) is very low for nearly three seasons (July–January), due to freshwater input from summer monsoon rainfall and seasonal discharge from several major rivers, including the Ganga‐Brahmaputra‐Meghna and the Irrawady. The low‐salinity surface layer leads to a very shallow density mixed layer, profoundly influencing air‐sea interaction on diurnal to seasonal time scales. We use mooring and satellite observations to study the mechanisms responsible for lateral dispersal of river water in the north BoB on subseasonal time scales (days to weeks) during the summer monsoon season of 2013. A new ocean current data set, the BoB near‐surface current and advection estimation (BoBcat), is developed to account for the influence of near‐surface stratification on directly wind‐forced currents. A salt balance based on Aquarius sea surface salinity, BoBcat currents and moored observations shows that subseasonal sea surface salinity variability is mainly due to lateral advection. A shallow Ekman current, forced by enhanced wind stress during an active spell of the monsoon in mid‐August, plays a dominant role in moving freshwater from the western boundary to the interior. During subseasonal spells of weak monsoon winds, stirring by mesoscale eddies is the main mechanism of dispersal of low‐salinity water. The dispersal of river water in the BoB is very sensitive to surface wind stress.