Formation of Abyssal Downwelling-Favorable Prograde Flows via Mesoscale Eddy Potential Vorticity Mixing

Abstract Observations and simulations have revealed widespread prograde mean flows (along-isobath currents in the direction of topographic wave propagation) over sloping topography in the abyssal ocean. While their emergence is consistent with quasi-two-dimensional turbulence theory, the factors governing their spatial variability remain poorly understood. Using a suite of eddy-resolving, process-oriented simulations, we investigate the driving mechanisms and parameter dependence of abyssal prograde flows. Consistent with previous studies, we find that prograde flows are driven by eddy momentum fluxes arising from eddy stirring of potential vorticity (PV), gradients of which are associated with the sloping seafloor. Moreover, we identify a nonmonotonic dependence of prograde flow strength on bottom steepness: Prograde flow initially strengthens with bottom slope but weakens once the slope becomes sufficiently steep, contradicting predictions from classical quasi-two-dimensional turbulence theories. Diagnostics of eddy PV diffusivity reveal that eddy PV stirring becomes much less efficient over steep slopes, where eddies propagate much faster relative to the background flow and tracer filaments are advected out of the eddy before significant stirring occurs. The suppressed eddy PV diffusivity results in reduced PV mixing despite larger topographic PV gradients over steep slopes. A scaling is proposed to constrain the eddy PV diffusivity, which, in turn, yields a scaling for prograde flow strength based on the leading-order momentum balance. The proposed scaling predicts stronger prograde flow with high eddy kinetic energy, intermediate slope Burger numbers, and weak friction. These findings improve our understanding of abyssal prograde flow distribution and highlight the joint role of topography and mesoscale eddies in regulating abyssal circulation. Significance Statement In the deep ocean, the sloping seafloor structures the pathways of large-scale currents, steering them to flow parallel to the slope with shallower (deeper) water to their right in the Northern (Southern) Hemisphere. Via the influence of Earth’s rotation, these currents produce secondary downhill flows along the seafloor and therefore may play an important role in shaping the pathways via which deep waters circulate back up to the ocean surface. Yet, how these deep currents depend on local environmental conditions remains poorly constrained. This study uses idealized simulation experiments and poses a novel theory to link key oceanic environmental parameters with the strength of deep, along-slope flows. These findings provide a key step toward assessing the impact of these currents on the global-scale circulation of deep-ocean waters.