Submesoscale instabilities in the Bay of Bengal: Results from frontal process studies during the winter monsoon

We analyze submesoscale signatures in the Bay of Bengal during the winter monsoon. Earlier observational and numerical studies have established the importance of submesoscale dynamics at mid-latitude fronts in O(100m) mixed layers. In this study we investigate the potential for these instabilities at fronts in shallow stratified boundary layers with strong horizontal and vertical density gradients. Such conditions are typical of the Bay during the winter monsoon, when the mixed layers are O(5-40m) and the lateral buoyancy gradients are O(10^-6s^-2) over scales O(1-10km). We use observations from three high-resolution process studies at density fronts, conducted during two cruises in November and December 2013. The process-study sites are in the central, northern and the southern Bay. The data spans a wide array of in-situ measurements, resolving lateral scales from O(10m) to O(10km), allowing us to map for the first time, the submesoscale variability of salinity, temperature and velocity in the Bay. The three process studies present a range of forcing conditions, thus providing a natural setting to contrast their effects on submesoscale mechanisms. Our analysis shows the submesoscale field is most active for the central-bay process study, forced with downfront winds and embedded within a confluent mesoscale field, the latter a known recipe for frontogenesis. We find the flow conditions during this survey satisfy the necessary conditions for forced symmetric instability, opening the possibility for energy pathways to smaller scales, previously unknown in the bay. The results presented here show lateral processes at submesoscales are important in the bay, even at locations far from the coast.