Dynamics of eddying abyssal mixing layers over sloping rough topography

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Date
2022-07-15
Authors
Drake, Henri F.
Ruan, Xiaozhou
Callies, Jörn
Ogden, Kelly
Thurnherr, Andreas M.
Ferrari, Raffaele
Version
First author draft
OA Version
Citation
H.F. Drake, X. Ruan, J. Callies, K. Ogden, A.M. Thurnherr, R. Ferrari. 2022. "Dynamics of eddying abyssal mixing layers over sloping rough topography" Journal of Physical Oceanography. https://doi.org/10.1175/jpo-d-22-0009.1
Abstract
The abyssal overturning circulation is thought to be primarily driven by small-scale turbulent mixing. Diagnosed watermass transformations are dominated by rough topography “hotspots”, where the bottom-enhancement of mixing causes the diffusive buoyancy flux to diverge, driving widespread downwelling in the interior—only to be overwhelmed by an even stronger up-welling in a thin Bottom Boundary Layer (BBL). These watermass transformations are significantly underestimated by one-dimensional (1D) sloping boundary layer solutions, suggesting the importance of three-dimensional physics. Here, we use a hierarchy of models to generalize this 1D boundary layer approach to three-dimensional eddying flows over realistically rough topography. When applied to the Mid-Atlantic Ridge in the Brazil Basin, the idealized simulation results are roughly consistent with available observations. Integral buoyancy budgets isolate the physical processes that contribute to realistically strong BBL upwelling. The downwards diffusion of buoyancy is primarily balanced by upwelling along the sloping canyon sidewalls and the surrounding abyssal hills. These flows are strengthened by the restratifying effects of submesoscale baroclinic eddies and by the blocking of along-ridge thermal wind within the canyon. Major topographic sills block along-thalweg flows from restratifying the canyon trough, resulting in the continual erosion of the trough’s stratification. We propose simple modifications to the 1D boundary layer model which approximate each of these three-dimensional effects. These results provide local dynamical insights into mixing-driven abyssal overturning, but a complete theory will also require the non-local coupling to the basin-scale circulation.
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Note: This work has not yet been peer-reviewed and is provided by the contributing author(s) via EarthArXiv.org as a means to ensure timely dissemination of scholarly and technical work on a noncommercial basis. Copyright and all rights therein are maintained by the author(s) or by other copyright owners. It is understood that all persons copying this information will adhere to the terms and constraints invoked by each author’s copyright. This work may not be reposted without explicit permission of the copyright owner.This work is under review at the Journal of Physical Oceanography. Copyright inthis work may be transferred without further notice. [YO note: same of "Diapycnal displacement, diffusion, and distortion ..."]