Stability of western boundary currents with an application to pulselike behavior of the Agulhas Current

The northern Agulhas Current off South Africa displays very steady flow cha racteristics. Roughly five to six times per year the flow is disrupted by t he passage of large solitary meanders, propagating downstream. Observations obtained from moored current meters show that these so-called "Natal Pulse s" extend over the full depth of the current, Several hypotheses have been proposed for the generation of these pulses. Here the authors investigate w hether they arise as a barotropic instability on the inshore flank of the c urrent itself. The stability analysis is performed using a prescribed backg round state, loosely based on Agulhas Current observations. Unstable inshor e modes are found with propagation speeds close to those of the observed pu lses. To explain the intermittent temporal behavior of the pulses, the inte raction of the unstable modes of the western boundary current in the nonlin ear regime is investigated. The results indicate that the onset of pulselik e behavior is found in wave-mean How interaction of unstable inshore modes with the background jet. Whether the observed timing of pulses is due to in ternal variability or set by processes external to Aguihas Current proper r emains an open question.