Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies

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Abstract

The transport of warm and salty Indian Ocean waters into the Atlantic Ocean—the Agulhas leakage—has a crucial role in the global oceanic circulation1 and thus the evolution of future climate. At present these waters provide the main source of heat and salt for the surface branch of the Atlantic meridional overturning circulation (MOC)2. There is evidence from past glacial-to-interglacial variations in foraminiferal assemblages3 and model studies4 that the amount of Agulhas leakage and its corresponding effect on the MOC has been subject to substantial change, potentially linked to latitudinal shifts in the Southern Hemisphere westerlies5. A progressive poleward migration of the westerlies has been observed during the past two to three decades and linked to anthropogenic forcing6, but because of the sparse observational records it has not been possible to determine whether there has been a concomitant response of Agulhas leakage. Here we present the results of a high-resolution ocean general circulation model7,8 to show that the transport of Indian Ocean waters into the South Atlantic via the Agulhas leakage has increased during the past decades in response to the change in wind forcing. The increased leakage has contributed to the observed salinification9 of South Atlantic thermocline waters. Both model and historic measurements off South America suggest that the additional Indian Ocean waters have begun to invade the North Atlantic, with potential implications for the future evolution of the MOC.

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Figure 1: Large-scale circulation changes south of Africa.
Figure 2: Thermocline changes in the Southern Hemisphere.
Figure 3: Increase of Agulhas leakage.
Figure 4: Inter-annual to decadal variability in the Agulhas Current.
Figure 5: Pathways of the Agulhas leakage into the North Atlantic.

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Acknowledgements

The integrations of the experiments have been performed at the Höchstleistungsrechenzentrum Stuttgart (HLRS) and the Computing Centre at Kiel University. We thank the NEMO, AGRIF and Ariane System Teams for their technical support. The analysis was supported by the DFG projects BO 907/2-2 and SFB 754 (TP A2, http://www.sfb754.de). J.R.E.L. received support from the International Bureau of the BMBF (SUA 07/004) and from the South African National Research Foundation.

Author Contributions A.B. and C.W.B. conceived the experimental concept. A.B. implemented and conducted the experiments, and carried out the analysis. F.U.S. performed the observational analysis. All authors discussed the results and jointly wrote the manuscript.

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Correspondence to A. Biastoch.

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Biastoch, A., Böning, C., Schwarzkopf, F. et al. Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies. Nature 462, 495–498 (2009) doi:10.1038/nature08519

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