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1. Dynamics of eddying abyssal mixing layers over sloping rough topography

   https://doi.org/10.1175/JPO-D-22-0009.1  

   Drake, Henri F.; Ruan, Xiaozhou; Callies, Jörn; Ogden, Kelly; Thurnherr, Andreas M.; Ferrari, Raffaele (July 2022, Journal of Physical Oceanography)

   Abstract 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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2. Mixing-Driven Mean Flows and Submesoscale Eddies over Mid-Ocean Ridge Flanks and Fracture Zone Canyons

   https://doi.org/10.1175/JPO-D-19-0174.1  

   Ruan, Xiaozhou; Callies, Jörn (January 2020, Journal of Physical Oceanography)

   Abstract To close the abyssal overturning circulation, dense bottom water has to become lighter by mixing with lighter water above. This diapycnal mixing is strongly enhanced over rough topography in abyssal mixing layers, which span the bottom few hundred meters of the water column. In particular, mixing rates are enhanced over mid-ocean ridge systems, which extend for thousands of kilometers in the global ocean and are thought to be key contributors to the required abyssal water mass transformation. To examine how stratification and thus diabatic transformation is maintained in such abyssal mixing layers, this study explores the circulation driven by bottom-intensified mixing over mid-ocean ridge flanks and within ridge-flank canyons. Idealized numerical experiments show that stratification over the ridge flanks is maintained by submesoscale baroclinic eddies and that stratification within ridge-flank canyons is maintained by mixing-driven mean flows. These restratification processes affect how strong a diabatic buoyancy flux into the abyss can be maintained, and they are essential for maintaining the dipole in water mass transformation that has emerged as the hallmark of a diabatic circulation driven by bottom-intensified mixing. 

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3. Enhanced vertical mixing in the glacial ocean inferred from sedimentary carbon isotopes

   https://doi.org/10.1038/s43247-021-00239-y  

   Wilmes, Sophie-Berenice; Green, J. A. Mattias; Schmittner, Andreas (August 2021, Communications Earth & Environment)

   Abstract Reconstructing the circulation, mixing and carbon content of the Last Glacial Maximum ocean remains challenging. Recent hypotheses suggest that a shoaled Atlantic meridional overturning circulation or increased stratification would have reduced vertical mixing, isolated the abyssal ocean and increased carbon storage, thus contributing to lower atmospheric CO₂concentrations. Here, using an ensemble of ocean simulations, we evaluate impacts of changes in tidal energy dissipation due to lower sea levels on ocean mixing, circulation, and carbon isotope distributions. We find that increased tidal mixing strengthens deep ocean flow rates and decreases vertical gradients of radiocarbon andδ¹³C in the deep Atlantic. Simulations with a shallower overturning circulation and more vigorous mixing fit sediment isotope data best. Our results, which are conservative, provide observational support that vertical mixing in the glacial Atlantic may have been enhanced due to more vigorous tidal dissipation, despite shoaling of the overturning circulation and increases in stratification. 

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4. The Global Overturning Circulation

   https://doi.org/10.1146/annurev-marine-010318-095241  

   Cessi, Paola (January 2019, Annual Review of Marine Science)

   In this article, I use the Estimating the Circulation and Climate of the Ocean version 4 (ECCO4) reanalysis to estimate the residual meridional overturning circulation, zonally averaged, over the separate Atlantic and Indo-Pacific sectors. The abyssal component of this estimate differs quantitatively from previously published estimates that use comparable observations, indicating that this component is still undersampled. I also review recent conceptual models of the oceanic meridional overturning circulation and of the mid-depth and abyssal stratification. These theories show that dynamics in the Antarctic circumpolar region are essential in determining the deep and abyssal stratification. In addition, they show that a mid-depth cell consistent with observational estimates is powered by the wind stress in the Antarctic circumpolar region, while the abyssal cell relies on interior diapycnal mixing, which is bottom intensified. 

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5. Transformation and Upwelling of Bottom Water in Fracture Zone Valleys

   https://doi.org/10.1175/JPO-D-19-0021.1  

   Thurnherr, A. M.; Clément, L.; St. Laurent, L.; Ferrari, R.; Ijichi, T. (March 2019, Journal of Physical Oceanography)

   Abstract Closing the overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling. Where and how buoyancy is gained and water is transported upward remain topics of debate, not least because the available observations generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we synthesize available observations indicating that bottom water is made less dense and upwelled in fracture zone valleys on the flanks of slow-spreading midocean ridges, which cover more than one-half of the seafloor area in some regions. The fracture zones are filled almost completely with water flowing up-valley and gaining buoyancy. Locally, valley water is transformed to lighter densities both in thin boundary layers that are in contact with the seafloor, where the buoyancy flux must vanish to match the no-flux boundary condition, and in thicker layers associated with downward-decreasing turbulence levels below interior maxima associated with hydraulic overflows and critical-layer interactions. Integrated across the valley, the turbulent buoyancy fluxes show maxima near the sidewall crests, consistent with net convergence below, with little sensitivity of this pattern to the vertical structure of the turbulence profiles, which implies that buoyancy flux convergence in the layers with downward-decreasing turbulence levels dominates over the divergence elsewhere, accounting for the net transformation to lighter densities in fracture zone valleys. We conclude that fracture zone topography likely exerts a controlling influence on the transformation and upwelling of bottom water in many areas of the global ocean. 

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