Abstract
Regions of the ocean near continental slopes are linked to significant vertical velocities caused by advection over a sloping bottom, frictional processes and diffusion. Oceanic motions at submesoscales are also characterized by enhanced vertical velocities, as compared to mesoscale motions, due to greater contributions from ageostrophic flows. These enhanced vertical velocities can make an important contribution to turbulent fluxes. Sloping topography may also induce large-scale potential vorticity gradients by modifying the slope of interior isopycnal surfaces. Potential vorticity gradients, in turn, may feed back on mesoscale stirring and the generation of submesoscale features. In this study, we explore the impact of sloping topography on the characteristics of submesoscale motions. We conduct high-resolution (1 km × 1 km) simulations of a wind-driven frontal current over an idealized continental shelf and slope. We explore changes in the magnitude, skewness and spectra of surface vorticity and vertical velocity across different configurations of the topographic slope and wind-forcing orientations. All of these properties are strongly modulated by the background topography. Furthermore, submesoscale characteristics exhibit spatial variability across the continental shelf and slope. We find that changes in the statistical properties of submesoscale motions are linked to mesoscale stirring responding to differences in the interior potential vorticity distributions, which are set by frictional processes at the ocean surface and over the sloping bottom. Improved parameterizations of submesoscale motions over topography may be needed to simulate the spatial variability of these features in coarser-resolution models, and are likely to be important to represent vertical nutrient fluxes in coastal waters.
Highlights
The transition between mesoscale and submesoscale motions is typically marked by the loss of geostrophic balance
While this study largely focuses on the pattern of surface turbulence, it is known that surface characteristics are linked to potential vorticity (PV) distributions in the fluid interior
We examine the modulation of surface turbulence characteristics related to wind-induced frontal currents formed over a topographic slope
Summary
The transition between mesoscale and submesoscale motions is typically marked by the loss of geostrophic balance. This is in turn accompanied by the generation of larger vertical velocities through ageostrophic circulations, which occurs for Ro ∼ O(1) [1]. Parameterizations of submesoscale dynamics are only being implemented in global general circulation models (GCMs) [7], largely based on process studies in more idealized model configurations. Regional GCMs that directly resolve submesoscale motions show a potential increase in total eddy kinetic energy (EKE) by a factor of two [2,8] as compared to simulations where these motions are not resolved. The contribution of submesoscales to regional EKE may exhibit seasonal cycles [9,10]
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