GEOMETRIC: a geometrically informed, energy constrained mesoscale eddy parameterisation
The global stratification and circulation, and 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 parameterisation framework for mesoscale eddies, termed GEOMETRIC, is proposed. The GEOMETRIC framework closes eddy buoyancy fluxes according to the standard Gent--McWilliams scheme, but with the eddy transfer coefficient constrained by the eddy energy field, provided through a prognostic eddy energy budget evolving with the mean state. The GEOMETRIC scheme is implemented and tested in models of varying complexity, from a zonally averaged two dimensional channel model to three dimensional sector in MITgcm and NEMO. It is found that coarse resolution models employing GEOMETRIC display broad agreement in the sensitivity of the circumpolar transport, meridional overturning circulation and eddy energy pattern to surface wind stress as compared with analogous reference calculations at eddy permitting resolutions; in particular, coarse resolution models display eddy saturation and display potential for eddy compensation. Outlooks for NEMO and further research directions are discussed. 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.