A thin layer of fresh water from summer monsoon rain and river runoff in the Bay of Bengal (BoB) has profound influence on air‐sea interaction across the south Asian region, but the mechanisms that sustain the low‐salinity layer are as yet unknown. Using the first long time series of high‐frequency observations from a mooring in the north BoB and satellite salinity data, we show that fresh water from major rivers is transported by large‐scale flow and eddies, and shallow salinity stratification persists from summer through the following winter. The moored observations show frequent 0.2–1.2 psu salinity jumps with time scales of 10 min to days, due to O(1–10) km submesoscale salinity fronts moving past the mooring. In winter, satellite sea surface temperature shows 10 km wide filaments of cool water, in line with moored data. Rapid salinity and temperature changes at the mooring are highly coherent, suggesting slumping of salinity‐dominated fronts. Based on these observations, we propose that submesoscale fronts may be one of the important drivers for the persistent fresh layer in the north BoB.
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
Evaluating the submarine groundwater discharge (SGD) derived strontium (Sr) flux from the Bengal Basin to the Bay of Bengal (BoB) and determining its isotopic composition is crucial for understanding the marine Sr isotopic evolution over time. Measurements of spatially and temporally distributed water samples collected from the BoB show radiogenic 87Sr/86Sr, high Sr, calcium (Ca) concentrations and high salinity in samples collected dominantly from 100–120 m depth, which can be explained only by the contribution of saline groundwater from the Bengal Basin. These results provide a direct evidence of the SGD-Sr flux to the BoB. This SGD-Sr flux is however, spatially heterogeneous and using conservative hydrological estimates of the SGD flux to the BoB, we suggest a SGD Sr flux of 13.5–40.5 × 105 mol/yr to the BoB. Mass balance calculations using Sr concentrations and 87Sr/86Sr suggest up to 7% contribution of SGD to the 100–120 m BoB water samples. The identification of SGD at 100–120 m depth also provides an explanation for the anomalous variations in barium (Ba) concentrations and the δ18O-salinity relationship in intermediate depths of the BoB.
Surface salinity in the Bay of Bengal (BoB) has a well-defined seasonal cycle, with highest open ocean salinity (generally in the range 33-34 psu) in spring (March-May) and lowest values (24-32 psu) during late summer and autumn (September-November)
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