Diagnosing Subgrid Mesoscale Eddy Fluxes With and Without Topography. (8th December 2019)

• Record Type: Journal Article
• Authors: Khani, Sina; Jansen, Malte F.; Adcroft, Alistair
• Is Part Of: Journal of advances in modeling earth systems. Volume 11:Number 12(2019)
• Page Start: 3995
• Page End: 4015
• Publication Date: 2019-12-08
• Subjects: Geological modeling -- Periodicals; Climatology -- Periodicals; Geochemical modeling -- Periodicals; 551.5011
• DOI: 10.1029/2019MS001721
• Languages: English
• ISSNs: 1942-2466

Abstract

Abstract: General circulation models use subgrid‐scale (SGS) parameterizations to represent the effects of unresolved mesoscale eddies on large‐scale motions. Most of the current SGS parameterizations are based on a theoretical understanding of transient eddies, where the mean flow is a temporal average. In this work, we use a spatial filtering analysis to better understand the scale‐dependent characteristics of the SGS fluxes. Specifically, we apply the filtering approach to diagnose SGS eddy volume fluxes and eddy velocity scales in a hierarchy of model configurations from a flat bottom channel to an idealized Southern Hemisphere. Importantly, SGS volume fluxes include significant contributions from standing meanders; unlike for transient eddies, the vertically integrated SGS volume flux does not necessarily integrate to 0. To accommodate net vertically integrated eddy fluxes, we define a SGS eddy diffusivity based on planetary potential vorticity (PV) diffusion. We diagnose the transient and standing contributions to SGS fluxes and associated effective diffusivities. In the presence of bottom topography or continental barriers the standing component of the PV diffusivity becomes dominant at large filter scales in the westerly wind region, while the transient component remains dominant in the easterly wind region. Our results suggest that the diagnosed PV diffusivity can be parameterized using mixing length theory based on a priori estimates of SGS velocity and length scales. Key Points: Subgrid mesoscale eddy fluxes and velocity scales are diagnosed as a function of length scale using a spatial filtering approach In the presence of topography, subgrid‐scale (SGS) eddy fluxes contain a strong standing component SGS eddy fluxes are modeled as a diffusion of planetary potential vorticity, and an empirical model of diffusivity is constructed