Evaluating eddy mixing coefficients from eddy-resolving ocean models: A case study

A numerical model of a baroclinically unstable jet in a zonally periodic ch annel is used to analyze mesoscale eddy fluxes and their relationship with the gradients of the mean flow. The quasigeostrophic approximation proves t he best way to calculate potential vorticity fluxes in the primitive equati on model. Away from the surface layers, eddy fluxes of potential density ar e consistent with advection by eddy-induced velocities v* and w* as suggest ed by Gent et al. (1995). Eddies mix potential vorticity along isopycnals, so that v* is related to the gradients of potential vorticity rather than p otential density as implicitly assumed by Gent et al. The mixing coefficien t for potential vorticity, associated with the advective component of the e ddy fluxes, is found to be similar to the mixing coefficient of tracer anom alies on isopycnals. Both show a maximum at mid-depth below the jet core. T he present calculations support the analysis of Treguier, Held and Larichev (1997) and encourage further attempts to derive a parameterization based o n true mixing of potential vorticity.