It is proposed to investigate the physical processes controlling the low-frequency variations of the Agulhas Current retroflection and ring formation. This variability is caused by internal processes and external (to the ACS) influences. Methods from dynamical systems theory will be used to study the internal variability of the ACS and to develop a new dynamical theory of retroflection and ring formation. Output from a high horizontal (1/32) resolution Navy Layered Ocean Model (NLOM) simulation over the years 1979-2002 will be used to evaluate the importance of internal dynamics on the low-frequency variability of the ACS (versus external causes). This study is motivated by recent observations showing the existence of strong low-frequency (interannual-to-decadal) variability of the upper layer transport in the Agulhas Current retroflection region, and suggesting that there is a strong correspondence between maximum transport values and ring shedding events. It is also motivated by recent paleoceanographic proxy analyses indicating that during a glacial period, the Agulhas rings in- flux was significantly reduced. The influx, however, dramatically increased towards the end of the glacial period. It has been suggested that, due to salt anomalies carried by the rings, this increase of influx contributed to the re-start of deep water formation in the Atlantic. Although there are presently separate theories for both retroflection and ring formation, there is no satisfying theory connecting both phenomena as well as explaining their interannual variability. Without answers to these questions, the theory on the behavior of the ACS will stay incomplete. The dynamical systems study of the Agulhas Current fits also a broader attempt to determine systematically the solution structure, in particular the transitions to complex behavior of those solutions, in a hierarchy of ocean models of western boundary currents.
