Stability of Western Boundary Currents with an Application to Pulselike Behavior of the Agulhas Current

Abstract

The northern Agulhas Current off South Africa displays very steady flow characteristics. Roughly five to six times per year the flow is disrupted by the passage of large solitary meanders, propagating downstream. Observations obtained from moored current meters show that these so-called ?Natal Pulses? extend over the full depth of the current. Several hypotheses have been proposed for the generation of these pulses. Here the authors investigate whether they arise as a barotropic instability on the inshore flank of the current itself. The stability analysis is performed using a prescribed background state, loosely based on Agulhas Current observations. Unstable inshore modes are found with propagation speeds close to those of the observed pulses. To explain the intermittent temporal behavior of the pulses, the interaction of the unstable modes of the western boundary current in the nonlinear regime is investigated. The results indicate that the onset of pulselike behavior is found in wave?mean flow interaction of unstable inshore modes with the background jet. Whether the observed timing of pulses is due to internal variability or set by processes external to Agulhas Current proper remains an open question.