Barotropic instabilities of the Agulhas Current system and their relation to ring formation

Abstract

Recent work has shown that different steady retroflection regimes,a viscous and an inertial regime,exist for the Agulhas Current system in an idealized ge- ometry.In this paper,instabilities of these steady flows are considered by solving the linear stability problem numerically.Barotropic instabilities occur as so-called Hopf bifurcations in the viscous regime,with corresponding patterns related to Rossby basin modes.Depending on the value of the reduced gravity parameter, these instabilities may introduce intermonthly to interannual variability into the retroflection region.Finite amplitude development of these instabilities display "ring-like" localized anomaly patterns which travel around the tip of the continent. The results demonstrate that (i)the origin of the frequency of the ring formation is set by the physics of the large scale barotropic instabilities and (ii)the rectifi- cation processes due to these instabilities decrease the degree of retroflection of the mean state.The latter result suggests that the dominant mechanism of the retroflection is captured within the steady balances.