Special Physical Ocean Seminar, 19 Dec

Wednesday, December 19, 2018

Special Physical Oceanography Lunch Seminar
Subgrid-scale eddy fluxes in a hierarchy of models

Sina Khani
Princeton University, Program in Atmospheric and Oceanic Sciences

12:30 P.M., 425 Ocean Sciences Building


General Circulation models (GCMs) require subgrid-scale (SGS) parameterizations that represent the effects of unresolved eddies on large-scale motions. Current SGS models in GCMs are mainly based on theory for transient mesoscale eddies, while actual SGS eddy fluxes involve all motions that are not resolved by the grid spacing ∆, including SGS standing meanders that are governed by substantially different dynamics. Following the approach of large-eddy simulation, we introduce a spatial filtering analysis to better understand the characteristics of these scale-dependent SGS fluxes. The filtering approach is applied to a hierarchy of models from a flat bottom channel to an idealized Southern hemisphere in eddy-resolving simulations. For large filter widths, our results show that the role of SGS standing meanders are very important in the presence of bottom topography. In this situation, the SGS volume transport does not integrate to zero over a water column, resulting in an ill-defined Gent-McWilliams eddy parameterization in models with bottom topography. To make progress, we define an eddy diffusivity based on the planetary potential vorticity (PV). We decompose the subgrid-scale PV diffusivity into the transient and standing components. It is shown that the transient PV diffusivity increases with the filter width ∆f  and converges at the eddy scale ~4◦. Importantly, the standing PV diffusivity dominates the transient component when ∆f >1◦. Overall, our a priori study suggests that there is a transition from small-scale transient eddies to large-scale meanders and gyres in a scale-aware SGS eddy parameterization. The potential implications of these results are in running non-eddying climate models at ~1◦resolution or very low-resolution paleo-climate models at ~4◦ resolution. 

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