In this thesis the variability of the South Atlantic climate is investigated. In order to account for both large and regional scales, two ocean models are applied. An ocean general circulation model, forced with the National Centre for Environmental Prediction/ National Centre for Atmospheric Research reanalysis is used to investigate the South Atlantic leading modes and also to investigate the South Atlantic response to El Nino-Southern Oscillation induced climate variability. A regional ocean model is used to investigate variability in the South East Atlantic, especially in the Angola Benguela Frontal Zone.
A rotated Empirical Orthogonal Function analysis of the mixed layer temperature suggests a breakdown of the South Atlantic into four subdomains, with characteristic spatial and temporal scales: (a) the tropical Atlantic, with mainly interannual fluctuations, (b) the northeastern subtropics, with variability on an interannual to decadal scale, (c) the midlatitudes, with interannual and multi-decadal variability and (d) the southwestern subtropics/ midlatitudes with a mixture of interannual and decadal variability. These modes are closely connected to anomalous atmospheric circulation patterns, which induce typical forcing mechanisms for each region.

A regional ocean model is used to further investigate variability in the South East Atlantic. Several experiments are conducted in order to assess the sensitivity of the Angola Benguela Frontal Zone towards changes in windstress. The results suggest that the position of the Angola Benguela Frontal Zone is mainly determined by opposing northward and southward flow near the coast. The windstress curl plays a major role in altering the position of the Angola Benguela Frontal Zone, as it controls the southeastward flow of the South Equatorial Counter Current. The
opposing northward flow is mainly the result of geostrophic adjustment to coastal upwelling enforced by apparent alongshore windstress. The strength of the frontal zone is related to the strength of the alongshore windstress that controls the coastal upwelling. The internal variability of the Angola Benguela Front is investigated. It appears that anomalous northward (southward) positions of the Angola Benguela Frontal Zone are connected to anomalous intensities of the front. A mechanism is suggested whereby anomalous northward shifts of the Angola Benguela Frontal Zone are associated with an anticyclonic circulation pattern in the area that shifts the frontal system towards the north. This anomalous circulation also forces changes in coastal upwelling that in turn drive anomalous upper ocean temperatures, which act to enhance the meridional temperature gradient. This mechanism appears to be most efficient during austral spring and summer. Interannual variability within the front is investigated and is found to be connected to periodic westward propagating anomalies of the large scale sea surface height. It is suggested that oceanic instability processes are mainly responsible for this wave-like motion.
A comparison between internal variability near the Angola Benguela Frontal Zone and other modes of variability suggests that internal variability could at times account for the same amount of variability induced by El Nino-Southern Oscillation, and for up to 20% of the variability apparent during major warm events in the South East Atlantic such as Benguela Ninos.