Competition between mixed-layer instabilities in shallow fronts at subtropical latitudes in the ocean

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Much of the flow in the ocean, down to scales of around ten kilometers, is found to be near an equilibrium where horizontal pressure gradients are balanced by an acceleration that arises from the rotation of the Earth. These flows are mostly confined to the horizontal plane and are not very efficient at mixing the warm, lighter water near the surface with the colder, denser waters below. At smaller scales of a kilometer or so, the so-called submesoscale, this balance begins to break down and vertical motions become more energetic. Bulk of the existing literature on submesoscale physics has concentrated on fronts within deep, order 100 meter wintertime mixed layers. In this study, high-resolution large-eddy simulations will be used for a systematic study of sharp fronts in shallow, order 10 meter mixed layers and the instabilities capable of extracting the available potential energy from such fronts. These fronts are often found in subtropical latitudes. These high-resolution simulations will complement our existing knowledge of frontal instabilities in deep wintertime mixed layers. The simulations will also document the effects of surface waves on the onset of instabilities. Fronts are vital to the dissipation of the large-scale oceanic kinetic energy through small-scale turbulence. An improved understanding of frontal instabilities in shallow mixed layers, the theme of this project, contributes directly to our knowledge of the large-scale circulation. Frontal mechanisms also promote the supply of nutrients from deeper layers where they are abundant to the surface layers where they are consumed by planktonic plants. The project will also enable continued outreach efforts to convey the scope of this research to general audiences at the Ocean Explorium events in New Bedford, MA and to high school students through the Massachusetts Marine Educators Association.Shallow, salinity-controlled fronts are often generated in subtropical latitudes by local precipitation or the stirring of river runoff into filaments by large mesoscale eddies. The shallow mixed layers raise important questions regarding the likelihood of occurrence of various submesoscale instabilities observed and documented previously at fronts in deep mixed layers. Earlier observations in the wintertime Gulf Stream and numerical simulations of the same suggest the absence of a preference of one over the other. One of the central tasks in this project will be to examine whether the combination of a smaller Coriolis parameter in the subtropics and shallow mixed layers lead to a preference for symmetric instability over ageostrophic baroclinic instability. Such a preference has important implications as coarse-resolution climate models currently have a parameterization for baroclinic instability in the mixed layer but none for symmetric instability. This project also includes an analysis of the effects of Stokes drift, within the Craik-Leibovich framework, on the instabilities accompanying shallow mixed-layer fronts. Together, the set of proposed simulations have the potential to greatly enhance our knowledge of submesoscale frontal mechanisms in shallow mixed layers, thus building on our current knowledge of such mechanisms in deep wintertime mixed layers.