Oceanic Flow–Topography Interactions

Description:

This collection features observational, modeling, and theoretical work on the interaction of low-frequency oceanic flows with steep topography on time scales ranging from subtidal to mesoscale and seasonal. As flow goes around islands and submarine headlands, vorticity production leads to wakes and vortex streets. Flow over submarine ridges may generate lee waves. These processes provide routes for energy and momentum from the larger basin scales to (cascade to) small scales where turbulence occurs and energy is dissipated.

Collection organizers:

Gunnar Voet and Matthew H. Alford, Scripps Institution of Oceanography, University of California, San Diego

Oceanic Flow–Topography Interactions

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Jody M. Klymak

Abstract

Drag and turbulence in steady stratified flows over “abyssal hills” have been parameterized using linear theory and rates of energy cascade due to wave–wave interactions. Linear theory has no drag or energy loss due to large-scale bathymetry because waves with intrinsic frequency less than the Coriolis frequency are evanescent. Numerical work has tested the theory by high passing the topography and estimating the radiation and turbulence. Adding larger-scale bathymetry that would generate evanescent internal waves generates nonlinear and turbulent flow, driving a dissipation approximately twice that of the radiating waves for the topographic spectrum chosen. This drag is linear in the forcing velocity, in contrast to atmospheric parameterizations that have quadratic drag. Simulations containing both small- and large-scale bathymetry have more dissipation than just adding the large- and small-scale dissipations together, so the scales couple. The large-scale turbulence is localized, generally in the lee of large obstacles. Medium-scale regional models partially resolve the “nonpropagating” wavenumbers, leading to the question of whether they need the large-scale energy loss to be parameterized. Varying the resolution of the simulations indicates that if the ratio of gridcell height to width is less than the root-mean-square topographic slope, then the dissipation is overestimated in coarse models (by up to 25%); conversely, it can be underestimated by up to a factor of 2 if the ratio is greater. Most regional simulations are likely in the second regime and should have extra drag added to represent the large-scale bathymetry, and the deficit is at least as large as that parameterized for abyssal hills.

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