Comparison of near‐surface mean and eddy flows from two numerical models of the South Atlantic Ocean

Abstract

The near‐surface mean and eddy flows simulated by two eddy‐permitting models of the South Atlantic (S‐coordinate Primitive Equation Model (SPEM) and Ocean Parallel (OPA) model) are compared. These models are intrinsically different by virtue of their vertical discretizations (σ versus geopotential coordinates, respectively), so that their solutions differ essentially through the representation of current‐topography interactions. The path of the Agulhas Current, the generation process, the characteristics, and the subsequent trajectory of the Agulhas rings are differently affected by topography in the two models. Along the American western boundary, SPEM traps a larger part of the Antarctic Circumpolar Current (ACC) around the Falkland Plateau and consequently produces a stronger Malvinas Current, which overshoots far to the north, inshore of a rather realistic Brazil Current and Confluence regime. In contrast, OPA allows a large part of the ACC to flow across the Scotia Sea, produces a weaker Malvinas Current, traps the Brazil and Malvinas Currents along the American coast, and tends to distort the complex dynamics of the Confluence region and the Zapiola anticyclone. Some of these differences can be attributed to the particularities of each vertical coordinate system and to the representation of topographic slopes (staircases in OPA and facettes in SPEM). More generally, the topographic constraint (trapping of boundary currents and interaction of mean and eddy flows with bathymetry) is stronger in SPEM than in OPA. The topographic smoothing usually performed in sigma coordinate models removes mesoscale topographic roughness and reduces topographic slopes. This treatment is probably responsible for several of the differences observed in the numerical solutions.