Sina Khani | Program in Atmospheric and Oceanic Sciences, Princeton University
Subgrid-scale mesoscale eddies in a hierarchy of models for ocean simulations
General circulation models (GCMs) employ subgrid-scale (SGS) parameterizations to represent the effects of unresolved mesoscale eddies on large-scale motions, because it might not be computationally affordable to resolve a wide range of scales in the ocean. Most of the current SGS parameterizations in GCMs are based on the temporal mean-eddy decomposition, while SGS models require to parameterize all eddies that are not resolved by the grid spacing Δ. In analogy with the basis of large-eddy simulation, we introduce a spatial filtering approach to better understand the characteristics of these scale-dependent SGS fluxes. We apply the filtering approach to a hierarchy of models from flat-bottom channel to an idealized Southern-hemisphere section. Generally, the SGS thickness fluxes are parameterized using the eddy diffusivity models, such as the Gent-McWilliams (GM) parameterization. When the filter scale Δ_f is around or larger than 1°, our results show that standing meanders have a dominant role in SGS eddy fluxes in the presence of bottom topography. In this situation, the SGS volume transport does not integrate to zero over the water-column and that this makes GM parameterization become ill defined. We further suggest an eddy diffusivity model based on the planetary vorticity (PV) diffusion as an alternative closure for SGS thickness fluxes. Overall, in developing a scale-aware SGS eddy parameterization for ocean models, our a priori results show that there is a continuum of eddy scale, from small-scale transient eddies to basin-scale meanders and gyres.