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Abstract The emerging view of the abyssal circulation is that it is associated with bottom-enhanced mixing, which results in downwelling in the stratified ocean interior and upwelling in a bottom boundary layer along the insulating and sloping seafloor. In the limit of slowly varying vertical stratification and topography, however, boundary layer theory predicts that these upslope and downslope flows largely compensate, such that net water mass transformations along the slope are vanishingly small. Using a planetary geostrophic circulation model that resolves both the boundary layer dynamics and the large-scale overturning in an idealized basin with bottom-enhanced mixing along a midocean ridge, we show that vertical variations in stratification become sufficiently large at equilibrium to reduce the degree of compensation along the midocean ridge flanks. The resulting large net transformations are similar to estimates for the abyssal ocean and span the vertical extent of the ridge. These results suggest that boundary flows generated by mixing play a crucial role in setting the global ocean stratification and overturning circulation, requiring a revision of abyssal ocean theories.
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This content will become publicly available on February 1, 2027
Connecting the Local Response to Abyssal Mixing to the Basin-Scale Circulation
The circulation of the abyssal ocean is thought to be sustained by turbulence over a rough seafloor. Over sloping topography, this bottom-enhanced mixing produces diapycnal upwelling within a thin bottom boundary layer and downwelling aloft. Simplified theories have been developed to understand this local response to mixing, for example, by assuming an along-slope symmetry and imposing a constraint on the cross-slope transport. Ultimately, the local response to mixing on slopes must be connected to the basin-scale circulation; however, the barotropic transport must conserve potential vorticity. This coupling between the local response to mixing and the basin-scale circulation is studied here in the context of an idealized bowl-shaped basin using boundary layer theory and numerical solutions of the planetary geostrophic equations. In the absence of wind forcing and the joint effect of baroclinicity and relief (JEBAR), the leading-order barotropic transport is directed alongf/Hcontours, wherefis the planetary vorticity andHis the depth. The local response to mixing is coupled to this barotropic circulation. It can be thought of as simultaneously constrained by the barotropic circulation and forcing it via a bottom stress curl. Iff/Hcontours are closed, a strong barotropic circulation spins up along them as in simplified theories of the local response in the absence of along-slope variations. If these contours intersect the boundary, a case more typical in the real ocean, the barotropic transport is suppressed. This decouples the leading-order local response from the large-scale circulation and intensifies bottom boundary layer upwelling.
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- Award ID(s):
- 2149080
- PAR ID:
- 10685882
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- Volume:
- 56
- Issue:
- 2
- ISSN:
- 0022-3670
- Page Range / eLocation ID:
- 431 to 450
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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