Formation of interleaving layers in the Bay of Bengal

Abstract

Observations of the upper 200 m of the strongly stratified Bay of Bengal reveal a host of interleaving layers of watermass properties. The features appear as isopycnal spice anomalies and form multiple, parallel, ∼10 m thin layers, that are coherent over horizontal scales of 30–80 km. They are coincident with anomalies in density stratification and in some cases, with the vertical shear of horizontal velocity, but are not aligned with isopycnals. The cross-isopycnal slopes of these prominent features are O(10−4) and are, on average, 5 to 10 times smaller than the expected slopes of passive tracer anomalies due to mesoscale stirring. We present two alternate mechanisms by which such interleaving layers can be created from existing lateral spice gradients: (i) stirring by vertically sheared submesoscale eddies, and (ii) near-inertial wave shear. A numerical simulation of a density (and spice) front, based on observations of a narrow and fast jet, develops a submesoscale vortex that produces spice layering similar to our observations. The layering suggests that even a weak vertical diffusivity would greatly enhance the horizontal mixing of spice at submesoscales, the scales of the proposed processes, through shear dispersion. The analysis of the observations draws attention to the prevalence of submesoscale dynamics and NIWs with unusually high vertical wavenumbers in the Bay of Bengal.