The wind-driven circulation of the South Atlantic-Indian ocean — II. Experiments using a multi-layer numerical model

Abstract

A numerical modeling study of the circulation of the South Atlantic-Indian Ocean in a geometrically simplified domain is extended to include baroclinicity using the quasi-isopycnic coordinate model of Bleck and Boudra (1981, Journal of Physical Oceanography, 11, 755–770). Within this framework a number of model parameters are varied in an attempt to understand processes related to exchange of fluid between the two ocean basins. The importance of nonlinearity of the boundary currents is determined by varying mean upper layer depth among three experiments. Sensitivities of the model Agulhas retroflection to upper ocean stratification, lateral friction, presence of eastward drift and bottom drag are examined. The role of friction in the Indian Ocean western boundary layer is investigated and found to be important in separation from the boundary. Finally, horizontal resolution is doubled to resolve better the boundary layer and release of baroclinic instability. In advancing from the barotropic ( de Ruijter and Boudra, 1985, Dee-Sea Research, 32, 557–574), to the baroclinic model, an important new feature is development of an intense recirculation eddy just beyond the point where the Agulhas Current overshoots the tip of South Africa. The center of this recirculation becomes the pivoting axis of the model retroflection, and its intensity increases with increasing Rossby number. At the same time, less top layer water is exchanged between the basins. The retroflection region acts as a source-sink of available potential and kinetic energy for the Atlantic-Indian Ocean in the low Rossby number case. This source-sink is essentially shut off in the high Rossby number case. Energy is pumped into the bottom layer underneath the recirculation, however, and radiates westward in weak anticyclonic eddies. Similar to the one-layer case, the mechanism of the modeled retroflection is adjustment to a change in the vorticity balance as the Agulhas leaves the coast of Africa. Along that coast, the β-induced gain of relative vorticity is balanced by diffusion into the no-slip boundary. After separation, potential vorticity is essentially conserved and the gain of relative vorticity is manifested in an eastward turn. Agulhas ring formation in the model occurs only for certain parameter ranges, and is due to a closing of the retroflecting current onto itself. In the highly nonlinear case, interaction with the cold, low potential vorticity, eastward drift south of the retroflection area is also required for ring formation.