Observations and Dynamics of Low Salinity Cold Pools in the Bay of Bengal

An interesting feature observed during the first scientific cruise of the Air Sea Interactions in the Northern Indian Ocean (ASIRI) Research Initiative of the Office of Naval Research (2013-2017) was intermittently distributed Low Salinity Cold Water Pools (LSCWP) at the ocean surface. These observations were made in the Bay of Bengal (BoB), on board the Research Vessel Roger Revelle, operating out of Colombo, Sri Lanka. The first cruise leg of ASIRI was November 10 - 27, 2013, followed by two other cruise legs in December 2013 and two in July 2014. In LSCWP, the salinity was in the range (31.8-32.4) PSU and the temperature, (27.25-27.5) degrees Celsius, and they appear to be a mixture of river-influenced the BoB water from the north and the Arabian Sea water entering BoB from the south. Lateral stirring of two water types may have been the origin of these pools, but precipitation could also have some influence. Analysis of two LSCWP, on which detailed observations were made, had intriguing dynamic characteristics - one belonging to submesoscale and the other to mesoscale eddies ? inferred by the Rossby deformation radius calculations. The mesoscale LSCWP appeared to be in geostrophic equilibrium, bowl-shaped, partially-mixed and non-collapsing, whereas the submesoscale pool was dynamic, gravitationally collapsing and spreading, with spreading rate roughly the same as that of a gravity current front intruding into a stratified fluid. The LSCWP appear to have repercussions on air-sea interactions in the BoB, in that LSCWP dynamics may regulate the penetration of momentum and turbulent kinetic energy downward. Remarkably, the wind speeds on the mesoscale pool was almost twice that of the submesoscale pool, but the thermocline in the former was less active compared to the latter, as evident from gradient Richardson number and pycnocline displacement calculations. We hypothesize that robust, stable mesoscale LSCWP shield the penetration of air-sea fluxes downward due to the development of stratification within as well as by the presence of a very stable (high gradient Richardson number) interface at the bottom. At least over parts of the cruise, the thermocline in the BoB was replete with internal waves, shear instabilities, and entrainment events of different scales, and the extent to which LSCWP control overall mixing in the BoB is yet to be delineated.