On the dynamics of the Zapiola Anticyclone

Recent observations obtained in the South Atlantic suggest the existence of a strong anticyclonic flow positioned over a major bottom topographic feat ure known as the Zapiola Drift. Here a numerical simulation of the South At lantic is described in which this anticyclone is reproduced, and the model is used to diagnose the dynamics maintaining this flow. With a mean barotro pic transport of 140 Sv and bottom velocities of 10 cm s(-1), the simulated Zapiola Anticyclone compares well with in situ observations. Furthermore, the model surface eddy kinetic energy shows a local minimum over the drift, in agreement with observations from TOPEX/POSEIDON. Numerical experiments show that the circulation feature is sensitive to the intensity of the eddy field and to the particular value of the bottom friction. Both of these te ndencies are in agreement with a theoretical explanation of the Zapiola Ant icyclone that has recently appeared elsewhere. Thus we argue that the antic yclone is maintained by eddy-driven potential vorticity fluxes accelerating flow within topographically closed, ambient potential vorticity contours. As far as we know, this is the first South Atlantic simulation to reproduce the Zapiola Anticyclone in a realistic fashion. The quantitative success o f this experiment is attributed to the use of a topography following (sigma ) coordinate in a spatially well resolved model.