Dynamics of Agulhas Retroflection and Ring Formation in a Quasi-Isopycnic Coordinate Numerical Model

Abstract

Abstract The Agulhas retroflection region of the idealized South Atlantic-Indian ocean model described by De Ruijter and Boudra (1985) and Boudra and de Ruijter (1986) is analyzed in detail. First, the physical mechanism of the model retroflection is presented through illustration of the Agulhas' vorticity balance among various experiments, and second, the ring formation process is described in terms of its vertical structure and the associated energy conversions. Analysis of the vorticity balance in a one-layer model shows that both inertia and internal friction may account for a partial retroflection where a linear, weakly viscous system has none. In a series of one-, two-, and three-layer nonlinear, weakly viscous experiments, it is shown that retroflection is accomplished through a free inertial boundary layer, as suggested originally by De Ruijter (1982), and, furthermore, that it is the planetary vorticity advection, rather than the inertial overshooting distance, which is of greatest importance. Halving horizontal grid spacing (from 40 to 20 km) is shown to have a minor impact on the vorticity balance and the retroflection strength. The importance of a substantial viscous stress curl along the coast of Africa, as provided by the no-slip condition, is illustrated through comparison with a slippery Africa experiment. Because of the apparent importance of the planetary vorticity advection in the retroflection regime, an experiment with a more realistic South African coastal geometry is described. It is shown that the retroflection is still strong but that the associated recirculation is less intense. The primary terms of the vorticity equation have smaller magnitude but they exhibit the same basic balance as in the nonlinear, weakly viscous, rectangular Africa experiments: between planetary vorticity advection and viscous stress curl along the coast, and between planetary and relative vorticity advection on the seaward side of the coastal Agulhas and elsewhere throughout the retroflection. The mean energetics of the experiments are examined in order to gain additional understanding of the model retroflection. Also, the signatures of barotropic and baroclinic instability in the ring formation process for three experiments are studied in detail using eddy-mean energetics. Ring formation is accompanied by development of a pronounced cyclonic circulation in the lower layer. However, both barotropic and baroclinic conversions reach a maximum at the moment of ring cutoff. Therefore, a mixed instability is suggested.