Parameterizing mesoscale eddy buoyancy transport over sloping topography

Models that do not resolve the mesoscale eddies tend to parameterize their impacts such that the parameterized transport of buoyancy and tracers reduces the large-scale available potential energy and spreads tracers. However, the parameterizations used in the ocean components of current generation Earth System Models (ESMs) rely on an assumption of a flat ocean floor even though observations and high-resolution modelling show that eddy transport is sensitive to the potential vorticity gradients associated with a sloping seafloor. Using a hierarchy of model complexities, we show that (i) the buoyancy transport coefficient diagnosed from idealized eddy-resolving simulations is indeed reduced over bottom slopes (ii) such reduction can be skillfully captured by a mixing length parameterization by introducing the topographic Rhines scale as a length scale (iii) implementing such a modified `GM' parameterization in non-eddying simulations enhances the strength of thermal wind currents over the bottom slopes. 

 

Testing the new parameterization in realistic global coarse-resolution simulations shows that the impact of topography is most pronounced at high latitudes, enhancing the mean flow strength and reducing temperature and salinity biases. Reducing the buoyancy transport coefficient further with a mean-flow dependent eddy efficiency factor, has notable effects also at lower latitudes and leads to reduction of global mean tracer biases. We find that most of the tracer bias reduction follows from changing the buoyancy transport coefficient (GM), but we also discuss the impact of applying similar changes to the tracer mixing coefficient (Redi).