Tidally-Driven Diapycnal Upwelling in a Rough Sloping Canyon

Chih-Lun Liu, Henri Drake

Turbulent mixing over rough topography shapes abyssal ocean dynamics, yet a gap between large- and small-scale models underscores the need to connect processes across scales. Using three-dimensional large eddy simulations (LES) with quasi-realistic sloping topography from a Brazil Basin canyon, we force an ocean model solely with a barotropic M2 tide body force, allowing internal waves, instabilities, and turbulence to emerge naturally. With horizontally-periodic boundary conditions, bottom-intensified mixing homogenizes the water column and progressively suppresses water mass transformations. By introducing a mean slope, however, a restratifying cross-slope flow develops to balance this mixing, thus enabling the establishment of a non-trivial quasi-equilibrium state.
Subgrid scale turbulent mixing drives a water mass transformation that results in approximately 150 mSv of diapycnal upwelling within the bottom boundary layer (BBL) of a single sloping canyon. Extrapolated globally, such abyssal canyons could collectively contribute substantially to the deep branch of the overturning circulation.
Lagrangian particle tracking shows that most particles gain buoyancy and rise from their release positions, consistent with localized diapycnal upwelling near the sills. The correlation between buoyancy change and vertical displacement is strong near the sill, but weak downstream, where a hydraulically controlled overflow causes large vertical excursions with little buoyancy change. An analogous tracer release experiment shows that the tracer-weighted mean buoyancy increases at roughly twice the rate of the particle-ensemble mean, reflecting the diffusive drift that affects the tracer but not the particles.