The Deep Interior Downwelling, the Veronis Effect, and Mesoscale Tracer Transport Parameterizations in an OGCM

Abstract

Numerous numerical simulations of basin-scale ocean circulation display a vast interior downwelling and a companion intense western boundary layer upwelling at midlatitude below the thermocline. These features, related to the so-called Veronis effect, are poorly rationalized and depart strongly from the classical vision of the deep circulation where upwelling is considered to occur in the interior. Furthermore, they significantly alter results of ocean general circulation models (OGCMs) using horizontal Laplacian diffusion. Recently, some studies showed that the parameterization for mesoscale eddy effects formulated by Gent and McWilliams allows integral quantities like the streamfunction and meridional heat transport to be free of these undesired effects. In this paper, an idealized OGCM is used to validate an analytical rationalization of the processes at work and help understand the physics. The results show that the features associated with the Veronis effect can be related quantitatively to three different width scales that characterize the baroclinic structure of the deep western boundary current. In addition, since one of these scales may be smaller than the Munk barotropic layer, usually considered to determine the minimum resolution and horizontal viscosity for numerical models, the authors recommend that it be taken into account. Regarding the introduction of the new parameterization, diagnostics in terms of heat balances underline some interesting similarities between local heat fluxes by eddy-induced velocities and horizontal diffusion at low and midlatitudes when a common large diffusivity (here 2000 m2 s?1) is used. The near-quasigeostrophic character of the flow explains these results. As a consequence, the response of the Eulerian-mean circulation is locally similar for runs using either of the two parameterizations. However, it is shown that the advective nature of the eddy-induced heat fluxes results in a very different effective circulation, which is the one felt by tracers.