Acute sensitivity of ocean circulation to mesoscale eddy energy dissipation time-scale

Energetically constrained eddy parameterizations, where the eddy diffusivity or transfer coefficients contain an explicit dependence on the eddy energy together with an imposed parameterized eddy energy budget, have increasingly demonstrated their efficacy in reproducing aspects of high resolution models with explicit eddies in coarse resolution models with parameterized eddies. Here, results are presented for the GEOMETRIC parameterization scheme within the NEMO model using the ORCA1 global configuration, i.e. no resolution of mesoscale eddies anywhere, with eddy buoyancy fluxes parameterized by a down-gradient closure and the eddy transport coefficient constrained by the magnitude of total eddy energy. Employing GEOMETRIC results in the coarse resolution model displaying reduced sensitivity of the modeled circulation to changes in wind forcing, as compared to existing mesoscale parameterizations; notably, the phenomena of eddy saturation and (to some degree) eddy compensation are recovered. It is further demonstrated here that the ocean circulation exhibits an acute sensitivity to the mesoscale eddy energy dissipation time-scale: a roughly 50% change about a control mesoscale eddy energy dissipation time-scale leads to changes in the circumpolar transport, meridional overturning circulation and global ocean heat content that is comparable to equivalent experiments that halve/double the Southern Ocean wind stress, with the changes observed arising from changing eddy energy dissipation attributed to changes in the global pycnocline depth. Given the to-be-quantified uncertainties associated with mesoscale eddy energy dissipation and, indeed, on the general modeling of the mesoscale eddy energy, this work highlights a need to combine theoretical, modeling and observational efforts in order to constrain ocean eddy energetics to support developments in energetically constrained eddy parameterizations.