Submesoscale instabilities in the ocean bottom boundary layer: A new pathway for energy dissipation

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One of the long standing questions in physical oceanography is how the energy input to the ocean circulation is eventually dissipated, a problem with far-reaching implications for both our basic understanding of ocean circulation and for developing accurate ocean models. Recently it has been suggested that instabilities in the bottom layer and topographic wakes provide an important source of turbulent mixing, however the dynamics and energetics of these instabilities remain largely unexplored. At relatively small scales, where the rotation of the Earth is no longer the dominant factor, called submesoscales, instabilities in the ocean bottom layer modify the extraction, and dissipation of energy from the flow, and hence may alter classic conceptions of bottom turbulence. This research will use theory and idealized high-resolution numerical models to determine how submesoscale instabilities near the bottom, over and downstream of topography, modify turbulent mixing. These instabilities are believed to be common feature near the bottom, but are poorly understood in this setting relative to the surface layers. Likewise, these processes are currently absent in most large-scale ocean models, underscoring the need to develop a quantitative understanding of their impact and dependence on controlling parameters. This understanding is central to problems such as how dense water is brought to the surface in the abyssal overturning circulation, and the exchange of nutrients and other biogeochemical tracers between the bottom and interior layers. Results of this work will improve our understanding, and eventual parameterization, of dynamics along topography, and in particular how these boundary layer processes modify the larger-scale ocean circulation and energetics. This project will also enable the training of a postdoctoral researcher, and support the establishment of the initial research group of a junior faculty member.Preliminary results suggest that submesoscale instabilities in the bottom boundary layer (BBL) open a new pathway for dissipating kinetic energy, and further modify the turbulent mixing of buoyancy near topography, believed to be a key aspect of how dense water is brought back to the surface in the abyssal overturning circulation. These results suggest that classic conceptions of how flow-topography interaction generates turbulence are incomplete, and the role of submesoscale instabilities needs to be considered. This project will investigate the energetics of baroclinic, symmetric, and centrifugal instabilities generated by flow along topography. Large-Eddy Simulation (LES) will be used to examine how these instabilities modify the partitioning and flux of energy between eddy and mean kinetic and potential energy terms, and the irreversible dissipation of kinetic energy and mixing of buoyancy. Reynolds-averaged simulations with variable topography will also be used, in conjunction with the LES, to determine how the upstream BBL evolution influences the energetics of topographic wakes. This work will provide a more complete description of the physical processes, and parameter dependencies, which determine how submesoscale instabilities affect the energetics of the ocean circulation.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.