Competition between turbulent upwelling and downwelling in a sloping Submarine Canyon

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 large eddy simulations (LES) with quasi-realistic sloping topography from a Brazil Basin canyon, we force the model with only barotropic M2 tide, allowing internal waves, instabilities, and turbulence to emerge naturally. The sloping seafloor is crucial, enabling restratification that balances turbulent mixing to achieve a quasi-steady equilibrium state. A robust upslope mean current emerges only when the seafloor is tilted. Lagrangian particle tracking reveals a hotspot of net positive buoyancy change near the sill, reflecting strong diapycnal mixing. Diapycnal mixing drives a water mass transformation that results in approximately 150 mSv of upwelling within the bottom boundary layer (BBL) of a single canyon. Extrapolating this result to the global submarine canyons suggests that such localized mixing could collectively play a significant role in the global abyssal circulation. In contrast, the flat-bottom case lacks restratification, leading to persistent homogenization of the bottom layer.

The mean flow that emerges is oriented locally downhill near the seafloor, particularly at the canyon sill and over supercritical slopes at the flanks, driven by downhill jets and downward-reflecting internal wave beams. Negative vertical velocity occurs near critical and supercritical slopes, correlating with enhanced turbulence via shear and convective instabilities. During the upslope tidal phases, destratification triggers convective mixing. During the subsequent downslope phase, restratification requires transporting less buoyant water from above, a process limited by the short tidal timescale, and insufficient time for significant adjustment. Consequently, most canyon regions exhibit near-zero or negative time-mean stratification. Negative correlations between near-bottom vertical velocity and buoyancy flux divergence reveal how topographic slopes modulate diapycnal mixing. These findings refine our understanding of abyssal circulation by extending a new theoretical paradigm of competing upwelling and downwelling into the turbulent regime, offering critical insights into ocean energy pathways.