Space-time variability of near-surface salinity in the Bay of Bengal
Sree Lekha, J
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Freshwater from monsoon rain and rivers leads to a 5-10 m deep low-salinity layer in the north Bay of Bengal from August to February. The thin fresh layer, with strong stratification at its base, is highly responsive to air-sea momentum and heat flux. Moored observations at 18N, about 500 km away from major river mouths, show a 3-8 psu drop in surface salinity within a week as water from the Ganga-Brahmaputra-Meghna (GBM) river arrives at the mooring in late August-early September each year, and from the Irrawady river in November-December. In conjunction with satellite sea surface salinity (SSS) and surface currents, the moored observations indicate that dispersal of river water in the open ocean is mainly driven by the flow in mesoscale (order 100 km) eddies during calm phases of the summer monsoon, and by a swift, shallow wind-driven Ekman flow as monsoon winds strengthen. Six years of moored observations at 18N 89.5E show that surface salinity has a distinct quasi-biweekly (10-25 day) variability, which is not due to changes in freshwater input. Rather, changes in salinity are related to variations in surface winds associated with the quasi-biweekly mode of the Asian summer monsoon. During the active phase of the monsoon, a shallow wind-driven Ekman flow disperses river water to the north and east, leading to increased salinity at the moorings, and a rise of coastal sea level by 0.3-0.6 m within days along the eastern boundary. In situ and satellite observations show that the response of sea surface temperature (SST) to quasi-biweekly variations of surface heat flux is enhanced by a factor of two because the mixed layer is very shallow within the pool of river water, thus revealing a direct link between SST and surface salinity. During research cruises of ORV Sagar Nidhi in August-September 2014 and 2015, upper ocean temperature (T), salinity (S) and ocean currents (V) in the Bay of Bengal were measured with 0.5-1.5 km horizontal resolution and 1-2 m vertical resolution in order to study sub-mesoscale (1-10 km) variability. Underway CTD data show numerous sub-mesoscale salinity-dominated surface density fronts. The spatial scale of 30 major fronts lies in the range 3-25 km, and net density change across the fronts exceeds 0.3 kg/m3. An east-west asymmetry in isopycnal slope is due to Ekman flow, which drives relatively saltier, denser water over lighter water on the western side. Ship-borne ADCP measurements show that flow at sub-mesoscale fronts has Rossby number of order one. Of the 30 fronts, two are associated with swift 5-10 km wide jets in the upper 20 m. Mixed layer depth is shallower at the fronts than on either side, and is less than 10 m if lateral density gradient exceeds 0.1-0.2 kg/m3 per km. The observations indicate that slumping of sub-mesoscale salinity-dominated fronts is an important mechanism sustaining near-surface stratification in the north Bay of Bengal. Finally, basin-scale diapycnal diffusivity is estimated from freshwater balance within a control volume bounded by the 1018 kg/m3 isopycnal - T, S and V are from an eddy-permitting daily ocean analysis, and rainfall, evaporation and runoff from a continental runoff dataset and satellite observations. The amount of pure freshwater in the control volume increases from June to November each year due to net input from rain and runoff, and decreases from December to May. Water lighter than 1018 kg/m3 is not transported across the southern boundary of the Bay of Bengal, implying that the freshwater lost from the control volume is mixed to deeper layers within the basin. The freshwater balance indicates that average diapycnal diffusivity across the 1018 kg/m3 isopycnal surface in winter is nearly 5x10-5 m2/s, 3-5 times higher than in spring or summer. Winter mixing in the upper ocean is highest during episodes of cool, dry surface air, leading to enhanced evaporation and surface buoyancy loss.