Upper layer thermohaline structure of the Bay of Bengal during the 2013 northeast monsoon

Abstract

The Bay of Bengal is forced by the South Asian monsoon and associated seasonally reversing winds and freshwater fluxes. Here, we summarize a high-resolution, basin-wide survey of the Bay of Bengal conducted during the 2013 northeast (winter) monsoon. The observed mixed layer waters were sourced from the eastern Bay; and, generally, the upper ocean temperature-salinity properties followed regional trends as determined by Argo. Intense lateral mixed layer gradients were seen throughout the Bay. The majority of the observed fronts were salinity-controlled with compensating temperature gradients. Similarly, salinity was typically the primary control on the vertical density stratification near the mixed layer depth. In contrast to the shallow, sharp halocline, temperature stratification was enhanced over a larger depth range with the base of the near-surface isothermal layer often located beneath the mixed layer, indicating the presence of a barrier layer. Finescale layers in vertical temperature and salinity stratification were traced over distances greater than 100 km in the horizontal. Within a ~week-long period, we observed a significant reduction in the intensity of gradients (both lateral and vertical) as well as a shift in the relative importance of salinity in setting the observed gradients. By the end of the 17-day cruise when the ship was in the southern Bay of Bengal, temperature was an equal or dominant contributor to lateral and vertical gradients. Stratification and mixed layer fronts evolved over a relatively short time scale, likely in response to strong atmospheric forcing either associated with a tropical cyclone, sustained northeast monsoon conditions, or a combination of the two. Resolution of the observed lateral gradients requires sampling at lateral scales O(1km) or smaller, suggestive of the importance of the submesoscale.