Air–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km-wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (∼300 km wide) that carries high-salinity water into BoB during the northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden–Julian oscillation (MJO) permit the study of multiscale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air–sea interactions control the ABL and upper-ocean processes.

The Woods Hole Oceanographic Institution surface mooring with ORV Sagar Nidhi in the background.

Near-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. SREE LEKHA ET AL. 6330

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