Abstract
AbstractThe global stratification and circulation, as well as their sensitivities to changes in forcing, depend crucially on the representation of the mesoscale eddy field in a numerical ocean circulation model. Here, a geometrically informed and energetically constrained parameterization framework for mesoscale eddies—termed Geometry and Energetics of Ocean Mesoscale Eddies and Their Rectified Impact on Climate (GEOMETRIC)—is proposed and implemented in three-dimensional channel and sector models. The GEOMETRIC framework closes eddy buoyancy fluxes according to the standard Gent–McWilliams scheme but with the eddy transfer coefficient constrained by the depth-integrated eddy energy field, provided through a prognostic eddy energy budget evolving with the mean state. It is found that coarse-resolution models employing GEOMETRIC display broad agreement in the sensitivity of the circumpolar transport, meridional overturning circulation, and depth-integrated eddy energy pattern to surface wind stress as compared with analogous reference calculations at eddy-permitting resolutions. Notably, eddy saturation—the insensitivity of the time-mean circumpolar transport to changes in wind forcing—is found in the coarse-resolution sector model. In contrast, differences in the sensitivity of the depth-integrated eddy energy are found in model calculations in the channel experiments that vary the eddy energy dissipation, attributed to the simple prognostic eddy energy equation employed. Further improvements to the GEOMETRIC framework require a shift in focus from how to close for eddy buoyancy fluxes to the representation of eddy energetics.
Highlights
Coarse-resolution models employing existing GM-based parameterizations are generally found to be more sensitive than eddypermitting models to changing surface winds, though models employing eddy transfer coefficients that vary in three spatial dimensions are less sensitive than those that employ an eddy transfer coefficient that is varying in two spatial dimensions or is spatially constant (e.g., Farneti et al 2015)
One observed phenomenon is that the circumpolar transport increases with the strength of the surface wind forcing in coarse-resolution models employing existing GM-based parameterizations, whereas little sensitivity is observed in the equivalent models with explicit eddies (e.g., Munday et al 2013; Farneti et al 2015)
Eddy saturation is generally found in models that at least partially resolve a mesoscale eddy field (e.g., Hallberg and Gnanadesikan 2006; Hogg and Blundell 2006; Hogg et al 2008; Farneti and Delworth 2010; Farneti et al 2010; Morrison and Hogg 2013; Munday et al 2013; Hogg and Munday 2014) but not in models in which eddies are parameterized by GM-based schemes (e.g., Munday et al 2013; Farneti et al 2015)
Summary
Accurate representation of the mesoscale eddy field and its feedback onto the mean ocean state is one of the most pressing challenges for ocean modeling, especially in the ocean circulation models used for climate prediction, Denotes content that is immediately available upon publication as open access. One observed phenomenon is that the circumpolar transport increases with the strength of the surface wind forcing in coarse-resolution models employing existing GM-based parameterizations, whereas little sensitivity is observed in the equivalent models with explicit eddies (e.g., Munday et al 2013; Farneti et al 2015) This lack of sensitivity is known as eddy saturation (Hallberg and Gnanadesikan 2001; Tansley and Marshall 2001) and was first predicted on theoretical grounds by Straub (1993). A further discrepancy between eddy-permitting and coarse-resolution models employing existing GMbased parameterizations is the reduced sensitivity of the time-mean residual meridional overturning circulation to changing wind forcing obtained in eddy-permitting models (e.g., Meredith et al 2012; Viebahn and Eden 2012; Morrison and Hogg 2013; Munday et al 2013; Hogg and Munday 2014; Farneti et al 2015).
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