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High resolution shipboard observations reveal the complex processes controlling the evolution and subduction of a cold and salty, dense filament in the Bay of Bengal (BoB). The filament, likely formed through coastal upwelling, was advected offshore by the mesoscale velocity field, and was brought adjacent to a shallow, low salinity mixed layer by mesoscale strain. The front that formed on the Eastern edge of the dense filament was observed to undergo both restratification and steepening, creating barrier layers, in response to evolving mesoscale and submesoscale convergence and divergences. Measurements and analyses indicate that the development of both small‐scale instabilities such as symmetric instability (SI) and slightly larger scale ageostrophic secondary circulation (ASC), acted in concert to subduct and stir surface heat into the interior. These mechanisms also drive the generation of temperature inversions from O (1 km) lateral temperature variability at the surface, demonstrating the need for model parameterizations at such spatial scales. Our results highlight the importance of submesoscale dynamics in creating and warming barrier layers, facilitating vertical heat exchanges, and setting sea surface temperature in the Bay of Bengal, a critical input to coupled atmosphere/ocean models of the southwest monsoon. Such complex dynamics should be considered in regions where salinity governs stratification and compensated waters stir. Plain Language Summary: Improving predictions for the monsoon seasons in the Bay of Bengal depends on the accuracy of the ocean's surface temperatures, which involve both one‐dimensional and three‐dimensional processes that impact the vertical structure of the upper ocean. In this study, we observed a cold and salty water mass that upwelled near the coast of India drift into the center of the Bay where it met ambient low salinity water to create surface lateral density gradients. High‐resolution shipboard observations revealed evidence of small‐scale physical processes as well as features typical of regions where the vertical structure is governed by salinity. The observations document the complexity of upper ocean heat content on scales less than 10 km, with impacts for sea surface temperature and couple ocean ‐atmosphere forecasting of the southwest monsoon. Key Points: The subduction of a dense filament in the Bay of Bengal suggests the role of submesoscale processes in setting upper ocean heat contentBarrier layers form at edges of fronts, sequestering heat from the surface and creating shallow mixed layers with lateral scales O (10 km)Small‐scale lateral temperature variability is exchanged vertically through instabilities, encouraging formation of temperature inversions [ABSTRACT FROM AUTHOR]

Lateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central Bay of Bengal during the winter monsoon are explored using high-resolution data from a cruise in November 2013. The observations are from a radiator survey centered at a salinity-controlled density front, embedded in a zone of moderate mesoscale strain (0.15 times the Coriolis parameter) and forced by winds with a downfront orientation. Below a thin mixed layer, often ≤10 m, the analysis shows several dynamical signatures indicative of submesoscale processes: (i) negative Ertel potential vorticity (PV); (ii) low-PV anomalies with O(1-10) km lateral extent, where the vorticity estimated on isopycnals and the isopycnal thickness are tightly coupled, varying in lockstep to yield low PV; (iii) flow conditions susceptible to forced symmetric instability (FSI) or bearing the imprint of earlier FSI events; (iv) negative lateral gradients in the absolute momentum field (inertial instability); and (v) strong contribution from differential sheared advection at O(1) km scales to the growth rate of the depth-averaged stratification. The findings here show one-dimensional vertical processes alone cannot explain the vertical stratification and its lateral variability over O(1-10) km scales at the radiator survey. [ABSTRACT FROM AUTHOR]