Monday 28 Nov 2016: Emergent eddy saturation from an energy constrained parameterisation
Dr Julian Mak - University of Edinburgh
Numerical ocean models of the Southern Ocean at mesoscale eddy permitting resolutions are generally seen to have a diagnosed mean circumpolar transport that is relatively insensitive to varying wind stress, a phenomenon termed eddy saturation. Baroclinic instability limits the isopycnal slopes, and a balance between steepening of isopycnal slopes by wind forcing and flattening of isopycnal slopes by eddy form stress results in a relatively unchanged mean stratification profile which, via thermal wind shear, leads to a relatively insensitive circumpolar transport. Coarse resolution models employing the Gent--McWilliams parameterisation with a constant Gent--McWilliams coefficient has been seen to have difficulty in reproducing this phenomenon. It is shown here that, in an idealised two-dimensional zonally average channel model, emergent eddy saturation is achieved by employing the Marshall et al. (2012) form of the Gent--McWilliams coefficient, which has a linear dependence on the eddy energy, with the eddy energy provided by a prognostic parameterised eddy energy budget coupled to the dynamical equations. A recipe for implementing this into a General Circulation Model is proposed.