Generation and Evolution of Natal Pulses: Solitary Meanders in the Agulhas Current

Abstract

Solitary meanders of the Agulhas Current, so-called Natal pulses, may play an important role in the overall dynamics of this current system. Several hypotheses concerning the triggering of these pulses are tested using sea surface height and temperature data from satellites. The data show the formation of pulses in the Natal Bight area at irregular intervals ranging from 50 to 240 days. Moving downstream at speeds between 10 and 20 km day21 they sometimes reach sizes of up to 300 km. They seem to play a role in the shedding of Agulhas rings that penetrate the South Atlantic. The intermittent formation of these solitary meanders is argued to be most probably related to barotropic instability of the strongly baroclinic Agulhas Current in the Natal Bight. The vorticity structure of the observed basic flow is argued to be stable anywhere along its path. However, a proper perturbation of the jet in the Natal Bight area will allow barotropic instability, because the bottom slope there is considerably less steep than elsewhere along the South African east coast. Using satellite altimetry these perturbations seem to be related to the intermittent presence of offshore anticyclonic anomalies, both upstream and eastward of the Natal Bight.