Submesoscale Signatures in Shallow Stratified Layers: Observations from the Bay of Bengal

We explore lateral submesoscale processes at shallow salinity fronts in the central Bay of Bengal during the winter monsoon using high-resolution data from a cruise in November 2013. The observations are from a radiator survey centered at a sharp salinity front, embedded in a zone of moderate mesoscale strain (0.15f) and forced by winds with a downfront orientation. Below a thin mixed layer, often shallower than 10m, we find lateral submesoscale processes play a central role in setting the vertical stratification and its lateral variability over O(1-10 km) scales. Our analysis shows several dynamical signatures indicative of submesoscale processes: (i) O(f) relative vorticities at O(1 km) scales; (ii) negative Ertel potential vorticity (PV); (iii) low-PV anomalies with O(1-10 km) lateral extent, associated with weak stratification and negative relative vorticity; (iv) alignment between isopycnals and contours of absolute momentum (symmetric instability) ; and (v) negative gradients in the absolute momentum field (inertial instability). The flow in several of the regions with negative Ertel PV satisfies the necessary conditions for forced symmetric instability. The generation of lateral variations in stratification and isentropic PV (PV on isopycnals) over O(1-10 km) scales are largely consistent with conservation of PV at a surface front within a frontogenetic mesoscale strain field. The low-PV anomalies occur where the vorticity (and PV) estimated on isopycnals and the isopycnal thickness are tightly coupled, varying in lockstep to conserve PV.