Ship-based Observations of a Salinity-dominated Density Front in the Bay of Bengal during August-September 2015

Sree Lekha Jarugula, Indian Institute of Science, Center for Atmospheric and Oceanic Sciences, Bangalore, India, Eric A D'Asaro, Applied Physics Laboratory University of Washington, Seattle, WA, United States, Shivaprasad. S, Indian National Centre for Ocean Information Services, Hyderabad, India, Dipanjan Chaudhuri, Indian Institute of Science, Centre for Atmospheric and Oceanic Sciences, Bangalore, India, Andrew Lucas, Scripps Institution of Oceanography, La Jolla, CA, United States, Robert A Weller, Woods Hole Oceanographic Institution, Woods Hole, MA, United States and Debasis Sengupta, Indian Institute of Science, Bangalore, India
Abstract:
The north Bay of Bengal receives 70-80% of its annual total freshwater supply (from river and rain) in the summer months June to October. Moored observations suggest that the low-salinity near-surface layer in the north Bay of Bengal persists for nearly three seasons. The presence of the shallow fresh near-surface layer makes the upper ocean highly responsive to the air-sea fluxes. In order to study the three-dimensional physical processes that maintain near-surface stratification, we conducted two research cruises of ORV Sagar Nidhi in August-September of 2014 and 2015. Slumping of lighter water under heavier water due to dynamical instability of submesoscale (order 1-10 km) fronts can create or renew vertical stratification. It is not known if this process is active in the presence of shallow mixed layers. In 2015, the US research vessel Roger Revelle worked alongside the Sagar Nidhi for about a week. Two separate two-ship experiments were conducted (Exp 1 and Exp 2, 4.5 and 2.5 days) in the vicinity of 17oN, with the Sagar Nidhi covering a total distance of 1150 km. The two ships made coordinated, repeated crossings of a 10 km wide, east-west oriented front with 1-2 psu cross-front salinity difference. In Exp 1, the two ships moved along 20-40 km long, parallel tracks separated by 2.5-5 km. In Exp 2, the ships did 10-40 km scale surveys following a salinity drifter deployed in the front. The layer of fresh, cool water to the north of the front is 10-15 m deep. A 15-30 m deep temperature inversion lies below the surface mixed layer. In both experiments, we find that the mixed layer is shallower directly under the front than on either side, suggesting active restratification. The mixed layer is about 4-7 m deep when the absolute spatial gradient of surface potential density exceeds 0.05 kg/m3/km and 10-22 m deep for gradients less than 0.05 kg/m3/km. Further analysis is continuing and we shall present more complete results at the meeting.
Back to: From Monsoons to Mixing: Coupled Ocean-Atmosphere Processes and Biogeochemical Response in the Indian Ocean I