Dynamics of eddying abyssal mixing layers over rough topography

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Authors

Henri Francois Drake , Xiaozhou Ruan, Raffaele Ferrari, Andreas M. Thurnherr, Kelly Ogden, Jörn Callies

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 upwelling in a thin Bottom Boundary Layer (BBL). These watermass transformations are significantly underestimated by one-dimensional sloping boundary layer solutions, suggesting the importance of three-dimensional physics. Here, we use a hierarchy of models to generalize this one-dimensional 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 canyon flanks and the surrounding abyssal hills. These flows are strengthened by the restratifying effects of submesoscale baroclinic eddies on the canyon flanks 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 one-dimensional boundary layer model which approximate each of these three-dimensional effects. These results provide \textit{local} dynamical insights into mixing-driven abyssal overturning, but a complete theory will also require the non-local coupling to the basin-scale circulation.

DOI

https://doi.org/10.31223/X5FC9N

Subjects

Applied Mathematics, Earth Sciences, Oceanography and Atmospheric Sciences and Meteorology, Physical Sciences and Mathematics

Keywords

abyssal circulation, ocean mixing, Turbulence, Submesoscale, canyon

Dates

Published: 2022-01-07 16:01

Last Updated: 2022-01-10 23:41

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License

CC BY Attribution 4.0 International

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