The Agulhas System as a Key Region of the Global Oceanic Circulation

Abstract

The oceans around southern Africa form a unique system, impacting the regional and global climate [1]. From the Indian Ocean to the Atlantic Ocean vigorous interoceanic exchange of warm and saline waters takes place that is subject to a complicated interplay between local dynamics and global embedment. Central element of the circulation around South Africa is the Agulhas Current [2] that flows poleward along the east coast, closely bound to the shelf at first, and subsequently overshoots the southern tip of Africa to abruptly turn back into the Indian Ocean. Part of the warm and saline waters with tropical Indian Ocean origin, the “Agulhas leakage” [3], flows into the Atlantic and forms the surface return flow of the global thermohaline circulation towards the North Atlantic [4]. The exchange takes place in a highly nonlinear manner, with mesoscale eddies being separated from the retroflecting Agulhas Current, which then strongly interact in the Cape Basin [5]. West of the Cape Basin, large Agulhas rings that have been formed [6] transport the anomalous warm and saline waters into the South Atlantic. In addition to its own dynamics, the Agulhas Current system is influenced by nonlinearities in the source regions: mesoscale eddies originating from the Mozambique Channel and east of Madagascar [7, 8] drift southward and cause the Agulhas Current to be displaced offshore of its mean position by more than 100 km. These solitary meanders (a.k.a. “Natal Pulses”) [9] rapidly propagate downstream triggering the timing of Agulhas rings [10, 11].