Atmosphere Ocean Science Colloquium

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Observations of sub-mesoscale shallow, salinity fronts in the Bay of Bengal during winter monsoon

Speaker: Amit Tandon, U Mass, Dartmouth

Location: Warren Weaver Hall 1314

Date: Friday, October 6, 2017, 11 a.m.

Synopsis:

Monsoons are the life force of South Asian agriculture and economy. The intra-seasonal variability in South Asian Monsoons is poorly predicted and has been traced to inadequate representation of ocean mixing and air-sea interaction in numerical ocean models. The Office of Naval Research supported an Air-Sea Interaction Regional Initiative (ASIRI, 2013-2017) to collaboratively address these with multiple USA institutions in collaboration with regional partners. We present a brief overview and introduction to ASIRI, and then focus on a process study carried out during the 2013 winter Monsoon. This study focused on lateral submesoscale observations of shallow salinity fronts in the central Bay of Bengal in November. 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