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Abstract Realistic computational simulations in different oceanic basins reveal prevalent prograde mean flows (in the direction of topographic Rossby wave propagation along isobaths; aka topostrophy) on topographic slopes in the deep ocean, consistent with the barotropic theory of eddy-driven mean flows. Attention is focused on the western Mediterranean Sea with strong currents and steep topography. These prograde mean currents induce an opposing bottom drag stress and thus a turbulent boundary layer mean flow in the downhill direction, evidenced by a near-bottom negative mean vertical velocity. The slope-normal profile of diapycnal buoyancy mixing results in downslope mean advection near the bottom (a tendency to locally increase the mean buoyancy) and upslope buoyancy mixing (a tendency to decrease buoyancy) with associated buoyancy fluxes across the mean isopycnal surfaces (diapycnal downwelling). In the upper part of the boundary layer and nearby interior, the diapycnal turbulent buoyancy flux divergence reverses sign (diapycnal upwelling), with upward Eulerian mean buoyancy advection across isopycnal surfaces. These near-slope tendencies abate with further distance from the boundary. An along-isobath mean momentum balance shows an advective acceleration and a bottom-drag retardation of the prograde flow. The eddy buoyancy advection is significant near the slope, and the associated eddy potential energy conversion is negative, consistent with mean vertical shear flow generation for the eddies. This cross-isobath flow structure differs from previous proposals, and a new one-dimensional model is constructed for a topostrophic, stratified, slope bottom boundary layer. The broader issue of the return pathways of the global thermohaline circulation remains open, but the abyssal slope region is likely to play a dominant role.
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This content will become publicly available on March 1, 2026
Diapycnal Upwelling Driven by Tidally Induced Mixing over Steep Topography
Abstract Diapycnal upwelling along sloping topography has been shown to be an important component of the abyssal overturning circulation. Theoretical studies of mixing-driven upwelling have mostly relied on a time-averaged description of mixing acting on a mean stratification which ignores the intermittency of mixing. Typically, these studies prescribed a time-invariant turbulent diffusivity profile motivated by scenarios where tidal currents encounter gentle topography with small-scale corrugations, leading to subsequent propagation and breaking of internal waves. Here, a different scenario is considered where a tidal current interacts with smooth but steep topography, a case often encountered near continental margins and troughs. The performed nonhydrostatic simulations resolve both the strong oscillatory shear that develops along the steep critical topography and the associated mixing events. Strong diapycnal mixing is observed during the upslope phase of the tidal flow when both the near-boundary stratification and shear are enhanced. During the downslope phase, strong overturning events do develop, but they are associated with weak stratification and less efficient diapycnal mixing. These results highlight that the temporal evolution of both shear and stratification play a key role in setting when diapycnal mixing and water mass transformation occur along steep topography. In contrast, over gentle topography, tidal shears do not become sufficiently large to generate strong local mixing for typical oceanographic parameters.
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- Award ID(s):
- 2343008
- PAR ID:
- 10581099
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- Volume:
- 55
- Issue:
- 3
- ISSN:
- 0022-3670
- Page Range / eLocation ID:
- 229 to 241
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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