Trapping of Momentum due to Low Salinity Water in the north Bay of Bengal
Dipanjan Chaudhuri, Indian Institute of Science, Centre for Atmospheric and Oceanic Sciences, Bangalore, India, Amit Tandon, University of Massachusetts, Dartmouth, Mechanical Engineering, Dartmouth, MA, United States, J. Thomas Farrar, Woods Hole Oceanographic Institution, Physical Oceanography, Woods Hole, MA, United States, Robert A Weller, Woods Hole Oceanographic Institution, Woods Hole, MA, United States, R Venkatesan, National Institute of Ocean Technology, Chennai, India, Shivaprasad. S, Indian National Centre for Ocean Information Services, Modelling and Ocean Observations Group, Hyderabad, India, Jennifer A MacKinnon, University of California San Diego, La Jolla, CA, United States, Eric A D'Asaro, Applied Physics Laboratory University of Washington, Seattle, WA, United States and Debasis Sengupta, Indian Institute of Science, Bangalore, India
Abstract:
We study the relation between near-surface ocean stratification and upper ocean currents (momentum) during the diurnal cycle and subseasonal “active-break cycle” of the summer monsoon in the north Bay of Bengal. We use time series of hourly observations from NIOT moorings BD08, BD09 and an INCOIS mooring near 18 N, 89 E in 2013, and data collected during two research cruises of ORV Sagar Nidhi in August-September 2014 and 2015. Our analyses are based on upper ocean profiles of temperature, salinity and density (from moorings and a shipborne underway conductivity-temperature-depth profiler), velocity (Acoustic Doppler Current Profiler), and surface forcing (meterology sensors on moored buoy and ship). Monsoon breaks are characterized by low rainfall, low wind speed (0-5 m/s) and high incident shortwave radiation, whereas active phases are marked by intense rainfall, high wind speed (8-16 m/s) and low incident sunlight. Our main findings are: (i) Net surface heat flux is positive (ocean gains heat) during break spells, and sea surface temperature (SST) rises by upto 1.5 C in 1-2 weeks. (ii) During breaks, day-night SST difference can reach 1.5C; mixed layer depth (MLD) shoals to 5m during day time, and deepens to 15-20 m by late night/early morning. (iii) During active spells, SST cools on subseasonal scales; MLD is deep (exceeding 20 m), and diurnal re-stratification is weak or absent. (iv) Once very low-salinity water (<30 psu) from rivers arrives at the moorings in late August, MLD remains shallow, and is insensitive 