Generation of submesoscale frontal eddies in the Agulhas current

Abstract

This study addresses the dynamics of the Agulhas inshore front in the submesoscale range upstream of 26 E. Submesoscale frontal eddies are observed in the vicinity of Port Elizabeth (26 E) from satellite images and in observations collected from underwater gliders. Using a submesoscale-resolving numerical model (dx ~ 0.75 km), we are able to simulate similar submesoscale eddies. Barotropic instability is confirmed as the generation mechanism by a one-dimensional linear stability analysis and an eddy kinetic energy budget. Kinetic energy is transferred from the mean flow to the eddies through the mean horizontal shear, which is a signature of barotropic instability. When the Agulhas Current is in a nonmeandering state, submesoscale eddy generation is a recurrent process which locally drives the front's variability. Along the front, the spatial variability of barotropic instability is shaped by the background strain. A large strain aligned with the frontal axis intensifies the frontal shear upstream of 28 E while a weakening of the strain allows for barotropic instability to be triggered downstream. Although an intermittent process, the barotropic instability shows a dominant period of variability comparable with the variability of the Agulhas Current and Undercurrent.