Dynamics of eddying abyssal mixing layers over sloping rough topography

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.

Boundary Upwelling of Antarctic Bottom Water by Topographic Turbulence

“…This is consistent with one-dimensional idealized boundary layer solutions for sloping bathymetry with parameters similar to ours, which suggest that diapycnal upwelling occurs in a layer of height O(50 m) (Holmes & McDougall, 2020). Similarly, the quasi-realistic Brazil Basin simulations of Drake et al (2022) with 6 m vertical resolution demonstrated marginally resolved diapycnal upwelling in a bottom boundary layer of O(10) m.…”

“…Similarly, the quasi‐realistic Brazil Basin simulations of Drake et al. (2022) with 6 m vertical resolution demonstrated marginally resolved diapycnal upwelling in a bottom boundary layer of O (10) m.…”

“…Our work constitutes the first realistic demonstration of how resolved submesoscale and internal wave‐driven mixing processes induce boundary upwelling of deep waters, as predicted by the recently put forward upwelling/downwelling paradigm (De Lavergne et al., 2016; Ferrari et al., 2016; McDougall & Ferrari, 2017). However, still higher vertical resolution on the order of meters is needed to satisfactorily resolve upwelling in the bottom boundary layer (Drake et al., 2022). We also demonstrated the need for hourly temporal resolution to capture the water mass transformation correctly, especially outside of the bottom grid cell.…”

“…A recent study by Drake et al. (2022) resolved the three‐dimensional physical processes leading to diapycnal upwelling, downwelling, and restratification near topography in a quasi‐realistic simulation of a canyon in the Brazil Basin. Using an idealized flow with an initially uniform stratification and a realistic topography, they showed that an imposed observationally‐based exponential diffusivity profile led to near‐boundary diapycnal upwelling by a three‐dimensional eddying submesoscale flow.…”

“…The result that the densest waters upwell close to topography, with downwelling occurring above, is, to our knowledge, the first verification of the upwelling/downwelling hypothesis in a realistic, submesoscale-and internal wave-resolving numerical model with an online diffusivity parameterization. A recent study by Drake et al (2022) resolved the three-dimensional physical processes leading to diapycnal upwelling, downwelling, and restratification near topography in a quasi-realistic simulation of a canyon in the Brazil Basin.…”