Submesoscale instabilities near the sea-floor and their effects on the ocean circulation and mixing

Stanford University, Stanford CA

Investigators

Abstract

Mixing processes in the ocean bottom boundary layer (BBL) are thought to exert a strong control on the general circulation. For example, the enhancement of turbulent mixing associated with breaking internal waves over topography has been suggested to play a central role in closing the deep branch of the meridional overturning circulation. Likewise, turbulent mixing in the BBL can result when large-scale currents flowing along sloping topography are decelerated by bottom friction. Importantly, these processes generate temperature and salinity fields in the BBL that are well mixed in the vertical but not in the horizontal, similar to fronts near the sea surface. This similarity suggests that many types of submesoscale (horizontal scales of 0.1 - 10 kilometers) instabilities, which to-date have been studied almost exclusively in the context of upper-ocean fronts, may also be active in the BBL. In the surface ocean, submesoscale processes are known to play an important role in restratifying the surface mixed layer, thereby counteracting turbulent mixing, and in the dispersal and transport of tracers such as pollutants, dissolved gases, and nutrients. Understanding the conditions under which submesoscale processes may be active in the BBL, as well as their effect on the circulation and mixing, therefore has potentially far-reaching implications for understanding the physics and biogeochemical properties of both deep and coastal oceans. This project will support a promising early career scientist who conceived and wrote the bulk of this proposal and new scientific results will be incorporated into outreach lectures and laboratory demonstrations for local high-school students. This project will use a combination of theory and idealized numerical modeling to study the dynamics of submesoscale instabilities in the BBL generated during the frictional spin-down of a current on a slope. Preliminary calculations suggest that BBLs over sloping topography can support a submesoscale baroclinic instability mode, a BBL counterpart to surface mixed-layer instabilities. These instabilities are fast-growing, and relatively insensitive to the topographic slope, suggesting they may be a common feature of the ocean BBL. Initial numerical simulations also suggest that these baroclinic instabilities can lead to vertical buoyancy fluxes which, under some conditions, may dominate other more widely-studied sources of vertical buoyancy fluxes, such as the breaking of internal waves at topography. The proposed research will expand upon these initial results, using linear stability analysis and idealized numerical modeling, to provide a complete exploration of the parameter dependence of BBL submesoscale instabilities, with a focus on turbulent fluxes of buoyancy and potential vorticity. Boundary layer dynamics for flow over idealized variations in topography, including channel flows, will also be considered. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.