A diagnosis of thickness fluxes in an eddy-resolving model

Output from an eddy-resolving model of the North Atlantic Ocean is used to estimate values for the thickness diffusivity kappa appropriate to the Gent and McWilliams parameterization. The effect of different choices of rotational eddy fluxes on the estimated kappa is discussed. Using the raw fluxes (no rotational flux removed), large negative values (exceeding -5000 m(2) s(-1)) Of kappa are diagnosed locally, particularly in the Gulf Stream region and in the equatorial Atlantic. Removing a rotational flux based either on the suggestion of Marshall and Shutts or the more general theory of Medvedev and Greatbatch leads to a reduction of the negative values, but they are still present. The regions where kappa < 0 correspond to regions where eddies are acting to increase, rather than decrease (as in baroclinic instability) the mean available potential energy. In the subtropical gyre, K ranges between 500 and 2000 m(2) s(-1), rapidly decreasing to zero below the thermocline in all cases. Rotational fluxes and kappa are also estimated using an optimization technique. In this case, vertical bar K vertical bar can be reduced or increased by construction, but the regions where K < 0 are still present and the optimized rotational fluxes also remain similar to a priori values given by the theoretical considerations. A previously neglected component (nu) of the bolus velocity is associated with the horizontal flux of buoyancy along, rather than across, the mean buoyancy contours. The nu component of the bolus velocity is interpreted as a streamfunction for eddy-induced advection, rather than diffusion, of mean isopycnal layer thickness, showing up when the lateral eddy fluxes cannot be described by isotropic diffusion only. All estimates show a similar large-scale pattern for nu, implying westward advection of isopycnal thickness over much of the subtropical gyre. Comparing nu with a mean streamfunction shows that it is about 10% of the mean in midlatitudes and even larger than the mean in the Tropics.