CoAuthors

Vincent Combes (Institut Mediterrani d’Estudis Avançats, Spain); Ted Strub (Oregon State University, Uni)

This study describes the oceanic circulation over the Agulhas Bank (AB), identifies its dynamical drivers and characterizes its exchanges with the open ocean. The analysis is based on the analysis of satellite altimeter data and numerical experiments. The mean circulation patterns produced by the model are consistent with satellite and in-situ observations. Seasonal changes are characterized by an increase of the outflow from the AB towards the Benguela upwelling region during spring and summer and an increase of the inflow from the northeastern region into the AB. During winter, there is an inshore displacement of the mid-shelf current flowing over the eastern portion of the bank, which returns to its original position during spring. Passive tracers, neutrally buoyant floats, and water mass balances characterize the exchanges between the AB and the open ocean. These diagnostics show vigorous entrainments of deep Agulhas Current waters in the bottom layer of the shelf. Largest mass entrainments occur in the northeast region and largest detrainments along the southernmost tip of the continent. Cross-shelf exchanges are highly variable, with entrainments inversely correlated to wind and Agulhas Current transport. Process-oriented experiments show that the Agulhas influence is largely restricted to the outer shelf region. Inclusion of wind and tidal forcing are essential to replicate the shelf velocity and water mass structure. Tides strengthen the cross-shelf temperature and salinity gradients generated by the onshelf intrusions of the Agulhas Current. Wind forcing enhances the cross-shelf exchanges, particularly in the northeastern region, which manifest as a cooling and freshening of the shelf.