Freshwater in the Bay of Bengal : its fate and role in air-sea heat exchange

Abstract

The strong salinity stratification in the upper 50–80 m of the Bay of Bengal affects the response of the upper ocean to surface heat fluxes. Using observations from November to December 2013, we examine the effect of surface cooling on the temperature structure of the ocean in a one-dimensional framework. The presence of freshwater adds gravitational stability to the density stratification and prevents convective overturning, even when the surface becomes cooler than the subsurface. This stable salinity stratification traps heat within subsurface layers. The ocean’s reluctance to release the heat trapped within these subsurface warm layers can contribute to delayed rise in surface temperature and heat loss from the ocean as winter progresses. Understanding the dispersal of freshwater throughout the bay can help scientists assess its potential for generating the anomalous temperature response. We use the Aquarius along-track surface salinity and satellite-derived surface velocities to trace the evolution and modification of salinity in the Lagrangian frame of water parcels as they move through the bay with the mesoscale circulation. This advective tracking of surface salinities provides a Lagrangian interpolation of the monthly salinity fields in 2013 and shows the evolution of the freshwater distribution. The along-trajectory rate of salinification of water as it leaves the northern bay is estimated and interpreted to result from mixing processes that are likely related to the host of submesoscale signatures observed during our field campaigns.