²Ñ°äÌý5479
Speaker
Sina Khani |ÌýProgram in Atmospheric and Oceanic Sciences, Princeton University
Title
Subgrid-scale mesoscale eddies in a hierarchy of models for ocean simulations
Abstract
General circulation models (GCMs) employ subgrid-scale (SGS)​Ìýparameterizations to represent the effects of unresolved mesoscale eddiesÌýon large-scale motions, because it might not be computationallyÌýaffordable to resolve a wide range of scales in the ocean. Most of theÌýcurrent SGS parameterizations in GCMs are based on the temporal mean-eddyÌýdecomposition, while SGS models require to parameterize all eddies thatÌýare not resolved by the grid spacing Δ. In analogy with the basis ofÌýlarge-eddy simulation, we introduce a spatial filtering approach toÌýbetter understand the characteristics of these scale-dependent SGSÌýfluxes. We apply the filtering approach to a hierarchy of models fromÌýflat-bottom channel to an idealized Southern-hemisphere section.ÌýGenerally, the SGS thickness fluxes are parameterized using the eddyÌýdiffusivity models, such as the Gent-McWilliams (GM) parameterization.ÌýWhen the filter scale Δ_f is around or larger than 1°, our results showÌýthat standing meanders have a dominant role in SGS eddy fluxes in theÌýpresence of bottom topography. In this situation, the SGS volumeÌýtransport does not integrate to zero over the water-column and that thisÌýmakes GM parameterization become ill defined. We further suggest an eddyÌýdiffusivity model based on the planetary vorticity (PV) diffusion as anÌýalternative closure for SGS thickness fluxes. Overall, in developing aÌýscale-aware SGS eddy parameterization for ocean models, our a prioriÌýresults show that there is a continuum of eddy scale, from small-scaleÌýtransient eddies to basin-scale meanders and gyres.