Observations of mesoscale eddies in the South Atlantic Cape Basin: Baroclinic and deep barotropic eddy variability

Abstract

Anticyclones and cyclones in the eastern South Atlantic are characterized based on data collected during January 2003 to March 2005, along a Jason 1 satellite altimeter ground track, as part of the Agulhas South Atlantic Thermohaline Transport Experiment. Large and small cyclones and anticyclones were ubiquitous in the deep ocean of the eastern South Atlantic, as well as in the upper ocean. Eddy structures jointly corotating in the upper and deep water column were common; most of the time (94%) these were not axially aligned as they copropagated. The Agulhas rings and cyclones that populate the region generally carry both a steric component (baroclinic) and a mass loading component (deep barotropic structure). Average translation speeds were 7.5 cm s−1 for baroclinic eddies and twice as fast for barotropic eddies, irrespective of polarity. Translation speeds were higher than advection by the mean background flow field. In addition, large mixed baroclinic‐barotropic rings crashed into the Agulhas Ridge and nearby seamounts and split into two or more parts. Some ring parts were also observed to fuse together. Deep cyclones, as well as interactions with topography, were observed to play a role in the fission process of Agulhas rings. These processes can increase the population of Agulhas rings and their remnant eddies, which took three pathways from the Agulhas and into the Cape Basin: (1) a deep pathway between the continental slope and Erica Seamount, (2) a shallower pathway over or near the Agulhas Ridge and Schmitt‐Otto Seamount, and (3) a deep seaward pathway around the Agulhas Ridge.