Description: The Agulhas region around South Africa is a key region of global climate and climate change. Under present climate conditions the Agulhas leakage from the Indian to the Atlantic Ocean feeds the bulk of the upper limb of the meridional overturning circulation (MOC) in the Atlantic Ocean, highly affected by the nonlinear constituents of the Agulhas Current system.To examine the role of the mesoscale processes in the mean flow in the Agulhas system, particularly in regard to the Agulhas leakage and its effect on the Atlantic MOC, an innovative ocean modeling program has been set up that utilizes new global model components and methodologies developed in international cooperation (DRAKKAR) based on a framework of the European model system NEMO. The model configuration involves a high-resolution grid of the greater Agulhas region nested into a coarse-resolution global ocean –sea-ice model forced by atmospheric conditions of the period 1958 –2004. Due to an effective “two-way” nesting approach this system for the first time allows to unravel, how the explicitly simulated mesoscale variability in the Agulhas dynamics feeds back to the global ocean.There is vast range of mesoscale –mean flow interactions in the Agulhas region. In the South East Madagascar Current offshore eddies do lead to different modes of the current extension, one favoring cyclonic flow into the Mozambique Channel, the other anticyclonic eddies drifting towards southwest. Eddies generated in the central Mozambique Channel introduce strong perturbations into the western boundary current systems off the African coast by triggering Natal Pulses, causing offshore displacements of the Agulhas Current which then lead to strong changes in the volume transport of the Agulhas Current and eventually to upstream retroflections of the current back into the Indian Ocean. The barotropic nature of the interplay with Mozambique eddies and Natal Pulses also affects the Agulhas Undercurrent leading to strong fluctuations similar to observed ones, raising the question what portion of the AgulhasUndercurrent is a coherent flow throughout the South Indian Ocean and what portion is virtually generated by passing Natal Pulses.The sequence of model experiments demonstrates that upstream perturbations have a vital effect on the mesoscale dynamics in the Agulhas retroflection area. A comparison of the reference model with a sensitivity experiment not including the Mozambique eddies shows that they are not only triggering the shedding of Agulhas rings but also lead to more realistic eddy structures in the Cape Basin and beyond. However, the presence of these upstream perturbations does not alter the mean Agulhas leakage, i.e, the net volume transport from the Indian to the Atlantic Ocean.The magnitude of the Agulhas leakage is quantitatively strongly dependent on the representation of Agulhas rings and other associated mesoscale processes in the retroflection area; there is a strong difference in the interoceanic transport between the high-resolution, nested model and the coarser, non-eddying model, the latter leading to higher, unrealistic transport values. While in the time-mean the bulk of this difference is modifying the horizontal circulation of the subtropical super-gyre rather than the Atlantic MOC, the mesoscale dynamics of the Agulhas regime appear as an important source of decadal variability in the MOC: An isolation of the effect of the mesoscale demonstrated that the Agulhas leakage acts as the source of low-frequency undulations in thermocline depth, a signal carried across the South Atlantic by Rossby waves and into the North Atlantic by wave processes along the American continental slope. The resulting signal in MOC transport gradually diminishes from south to north, but has an amplitude in the tropical Atlantic of comparable magnitude to the effect of subarctic deep water formation processes discussed in previous studies. It is evident that a proper representation of the mesoscale processes it vital for the correct interpretation of variations of the upper ocean transport across the equator, and even at subtropical latitudes in the North Atlantic where current monitoring efforts aim at a quantification of inter-annual MOC variations.