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

The near-surface mean and eddy flows simulated by two eddy-permitting model s of the South Atlantic (S-coordinate Primitive Equation Model (SPEM) and O cean Parallel (OPA) model) are compared. These models are intrinsically dif ferent by virtue of their vertical discretizations (sigma versus geopotenti al coordinates, respectively), so that their solutions differ essentialy, t hrough the representation of current-topography interactions. The path of t he Agulhas Current, the generation process, the characteristics, and the su bsequent trajectory of the Agulhas rings are differently affected by topogr aphy in the two models. Along the American western boundary, SPEM traps a l arger part of the Antarctic Circumpolar Current (ACC) around the Falkland P lateau and consequently produces a stronger Malvinas Current, which oversho ots far to the north, inshore of a rather realistic Brazil Current and Conf luence regime. In contrast, OPA allows a large part of the ACC to flow acro ss the Scotia Sea, produces a weaker Malvinas Current, traps the Brazil and Malvinas Currents along the American coast, and tends to distort the compl ex dynamics of the Confluence region and the Zapiola anticyclone. Some of t hese differences can be attributed to the particularities of each vertical coordinate system and to the representation of topographic slopes (staircas es 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 usu ally performed in sigma coordinate models removes mesoscale topographic rou ghness and reduces topographic slopes. This treatment is probably responsib le for several of the differences observed in the numerical solutions.