The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments--backed by palaeoceanographic and sustained modern observations--are required to establish firmly the role of the Agulhas system in a warming climate.

[1] The Humboldt Current System is the most productive of the eastern boundary currents. In the northern part, the Peru Current System (PCS) is located between 5°S and 20°S. Along the Peruvian coast, an equatorward wind forces a strong coastal upwelling. A high resolution model is designed to investigate the mean circulation, the seasonal cycle, and the mesoscale dynamics for the PCS. The model is able to reproduce the equatorward Peru Coastal Current (PCC), the Peru Chile Under-Current (PCUC) which follows the shelf break towards the pole, and the Peru-Chile Counter-Current (PCCC) which flows directly towards the south and veers to the west around 15°S. While the upper part of the PCUC is close to the surface and might even outcrop as a counter current, the bottom part follows f H isolines. The PCCC appears to be directly forced by the cyclonic wind stress curl. The model is able to produce the upwelling front, the cold water tongue which extends toward the equator and the equatorial front as described in the literature. Model seasonal changes in SST and SSH are compared to measurements. For the central PCS, model EKE is 10% to 30% lower than the observations. The model eddy diameters follow a strong equatorward increase. The injection length scales, derived from the energy spectra, strongly correlate to the Rossby radius of deformation, confirming the predominant role of baroclinic instability. At 3°S, the model solution appears to switch from a turbulent oceanic regime to an equatorial regime dominated by zonal currents.Citation: Penven, P., V. Echevin, J. Pasapera, F. Colas, and J. Tam (2005), Average circulation, seasonal cycle, and mesoscale dynamics of the Peru Current System: A modeling approach,

2 25 Abstract 26 27 The Agulhas Current plays a crucial role in the thermohaline circulation through its 28 leakage into the South Atlantic. Under both past and present climates, the trade winds and 29 westerlies could have the ability to modulate the amount of Indian-Atlantic inflow. 30Compelling arguments have been put forward suggesting that trade winds alone have 31 little impact on the magnitude of Agulhas leakage. Here, employing three ocean models 32 for robust analysis -a global coarse resolution, a regional eddy-permitting and a nested 33 high-resolution eddy-resolving configuration -and systematically altering the position 34 and intensity of the westerly wind belt in a series of sensitivity experiments, it is shown 35 that the westerlies, in particular their intensity, control the leakage. Leakage responds 36proportionally to the westerlies intensity up to a certain point. Beyond this, through the 37 adjustment of the large-scale circulation, energetic interactions occur between the 38 Agulhas Return Current and the Antarctic Circumpolar Current that result in a state 39 where leakage no longer increases. This adjustment takes place within 1 to 2 decades. 40Contrary to previous assertions, our results further show that an equatorward (poleward) 41 shift in westerlies increases (decreases) leakage. This occurs due to the redistribution of 42 momentum input by the winds. It is concluded that the reported present-day leakage 43 increase could therefore reflect an unadjusted oceanic response mainly to the 44 strengthening westerlies over the last few decades. 45 46 3