Effects of Bottom Friction on Nonlinear Equilibration of an Oceanic Baroclinic Jet

Abstract

Bottom friction is an important sink of energy in the ocean. Indeed, high-resolution ocean models need bottom friction to achieve a satisfactory kinetic energy level at equilibrium. However, bottom friction has also subtle and discriminating effects on the different energy transfers and therefore on the 3D structure of the flow, some of which have to be clarified. In this study, those effects on an unstable baroclinic jet are reexamined using a primitive equation model. As in previous studies using quasigeostrophic models, it was found that bottom friction strongly affects the barotropic mode whereas the baroclinic modes are weakly changed. The new result is that bottom friction yields a significant space-scale selection. Analysis of the dynamics reveals strong agreement with previous quasigeostrophic studies at the mesoscale in the interior but differences in the eddy field at small scales close to the surface. A rationalization of these results is proposed by a comparison with preceding atmospheric studies. It is shown that the ?barotropic governor? of James and Gray is not active in ocean simulations and that the scale selection induced by bottom friction is primarily induced by nonlinear interactions in the three-dimensional structure of the eddy field.