Submesoscale Signatures in the Bay of Bengal During Winter 2013 Monsoon

Motivated by observations in Japan East Sea, sub-mesoscale frontal structures and their interactions with winds were investigated in a series of theoretical and numerical studies by Leif Thomas and Peter Rhines, focusing on nonlinear-Ekman flow due to interaction of down front winds and ocean fronts. Further progress motivated observational studies in the North Atlantic in winter 2012 as part of the ONR sponsored LATMIX (Scalable Lateral Mixing and Coherent Turbulence) Directed Research Initiative. These observations, related theoretical progress and process simulations have revealed much about sub-mesoscale instabilities in mid-latitudes. Could such instabilities be important in shallow tropical mixed layers of the Northern Indian ocean?

Though they span similar latitudes in the Northern Indian Ocean, Bay of Bengal is a low salinity basin, especially compared to the Arabian sea due to freshwater fluxes from the summer and winter Monsoon precipitation and riverine inputs in the Bay. This freshwater input gets stirred by oceanic mesoscale eddies into sharp frontal gradients with relatively shallow mixed layers, which interact with monsoonal wind forcing and lead to instabilities and vertical mixing. The upper ocean salinity stratification in the Bay is therefore set by a complex interplay of one-dimensional and three-dimensional processes at sub-mesoscales and smaller scales. The upper ocean stratification in turn modulates the air-sea fluxes at various spatio-temporal scales, thus influencing Monsoons.

An international collaborative initiative that brings scientists from USA, India and Sri-Lanka is ongoing (2013-2017) to improve the understanding of the circulation, upper ocean dynamics, and air-sea interactions in the Bay of Bengal. One of the programs ASIRI-OMM (Air-Sea Interactions in the Northern Indian Ocean, supported by Office of Naval Research, USA; Ocean Mixing and Monsoons supported by Ministry of Earth Sciences, India) conducted observational studies during winter Monsoon (November-December 2013) and in pre-Monsoon phase (June-July 2014).

We report on three process studies carried out in the North, Central and South Bay of Bengal during oceanographic cruises during winter Monsoon 2013. The three process studies contrast regions with varying frontal gradients, strength and orientation of winds (upfront versus downfront), and mixed-layer depth. In the central-bay process study, where the fronts were strongest and the winds were mostly downfront, we found numerous O(1-10km) submesoscale filaments with O(f) relative vorticity. We also observed several O(1km) patches within the mixed layer with negative potential vorticity. These patches met the criteria for forced symmetric instability, as outlined in Thomas et al. (DSR, 2013). The survey in the northern bay, where the gradients were weaker and the winds were mostly upfront, did not exhibit a profusion of submesoscale features. This was also true of the southern bay, where the mixed layers were deepest and the orientation of the winds varied during the survey.