Generation and evolution of natal pulses: Solitary meanders in the AgulhasCurrent

Solitary meanders of the Agulhas Current, so-called Natal pulses, may play an important role in the overall dynamics of this current system. Several h ypotheses concerning the triggering of these pulses are tested using sea su rface height and temperature data from satellites. The data show the format ion of pulses in the Natal Eight area at irregular intervals ranging from 5 0 to 240 days. Moving downstream at speeds between 10 and 20 km day(-1) the y sometimes reach sizes of up to 300 km. They seem to play a role in the sh edding 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 th e Natal Eight. The vorticity structure of the observed basic flow is argued to be stable anywhere along its path. However, a proper perturbation of th e jet in the Natal Eight area will allow barotropic instability, because th e bottom slope there is considerably less steep than elsewhere along the So uth African east coast. Using satellite altimetry these perturbations seem to be related to the intermittent presence of offshore anticyclonic anomali es, both upstream and eastward of the Natal Eight.