How topographic smoothing contributes to diferrences between the eddy flows simulated by sigma- and geopotential-coordinate models

Abstract

The characteristics of the mesoscale turbulence simulated at a resolution of 1/3° by a sigma-coordinate model (SPEM) and a geopotential-coordinate model (OPA) of the South Atlantic differ significantly. These two types of models differ with respect to not only their numerical formulation, but also their topography (smoothed in SPEM, as in every sigma-coordinate application). In this paper, the authors examine how these topographic differences result in eddy flows that are different in the two models. When the topography of the Agulhas region is smoothed locally in OPA, as is done routinely in SPEM, the production mechanism of the Agulhas rings, their characteristics, and their subsequent drift in the subtropical gyre, are found to converge toward those in SPEM. Furthermore, the vertical distribution of eddy kinetic energy (EKE) everywhere in the basin interior becomes similar in SPEM and OPA and, according to some current meter data, becomes more realistic when mesoscale topographic roughness is removed from the OPA bathymetry (as in SPEM). As expected from previous process studies, this treatment also makes the sensitivity of the Agulhas rings to the Walvis Ridge become similar in SPEM and OPA. These findings demonstrate that many properties of the eddies produced by sigma- and geopotential-coordinate models are, to a significant extent, due to the use of different topographies, and are not intrinsic to the use of different vertical coordinates. Other dynamical differences, such as the separation of western boundary currents from the shelf or the interaction of the flow with the Zapiola Ridge, are attributed to intrinsic differences between both models. More generally, it is believed that, in the absence of a correct parameterization of current-topography interactions, a certain amount of topographic smoothing may have a beneficial impact on geopotential coordinate model solutions.