Baroclinic Eddy Fluxes over Continental Slopes: Insights from a Model Inter-Comparison

Numerical ocean models are increasingly capable of resolving mesoscale eddies. This parallels the growing observational evidence that ocean eddies play a crucial roles in the climate system via transport and mixing processes globally. Over continental slopes, eddies have been shown to impact water mass formations and large-scale mass and tracer transports by mediating cross-slope transfer. This study explores the dynamics of wind-forced baroclinic turbulence over continental slopes, placing particular emphasis on the representation of such flows in different model coordinate systems. Results are derived from a suite of eddy-resolving process simulations of an idealized continental shelf and slope in modelsthat employ geopotential coordinates (the MIT general circulation model), terrain-following coordinates (the Regional Ocean Modeling System), and isopycnal coordinates (the Hallberg Isopycnal Model).

In almost-identical configurations, all three models sustain an energetic baroclinic eddy field that is confined to the top few hundred meters over continental slope. Strong lateral eddy momentum fluxes transfer wind-input momentum off the continental slope toward the open ocean. The resulting mean along-slope jet is therefore displaced offshore of the wind stress maximum, as is the extraction of momentum at the ocean bed. Below the top few hundred meters of the water column over the continental slope, eddies tend to feed the APE by consuming EKE, leading to a local up-gradient buoyancy fluxes, which are not permitted in conventional approaches to parameterization of eddy transfer. Despite the almost identical configuration of the three models, there are substantial quantitative differences between the simulated flows, for example in the mean stratification, along-slope jet structure, and eddy kinetic energy. Comparison against simulations with the continental slope removed and replaced by a planetary vorticity gradient suggests that these inter-model differences are primarily attributable to the differing representations of sloping bottom, rather than other aspects of the model algorithms. Our findings lay a foundation for future efforts to develop a parameterization of eddy transfer over continental slopes.