Agulhas leakage dynamics affects decadal variability in Atlantic overturning circulation

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Abstract

Predicting the evolution of climate over decadal timescales requires a quantitative understanding of the dynamics that govern the meridional overturning circulation (MOC)1. Comprehensive ocean measurement programmes aiming to monitor MOC variations have been established in the subtropical North Atlantic2,3 (RAPID, at latitude 26.5° N, and MOVE, at latitude 16° N) and show strong variability on intraseasonal to interannual timescales. Observational evidence of longer-term changes in MOC transport remains scarce, owing to infrequent sampling of transoceanic sections over past decades4,5. Inferences based on long-term sea surface temperature records, however, supported by model simulations, suggest a variability with an amplitude of ±1.5–3 Sv (1 Sv = 106 m3 s-1) on decadal timescales in the subtropics6. Such variability has been attributed to variations of deep water formation in the sub-arctic Atlantic, particularly the renewal rate of Labrador Sea Water7. Here we present results from a model simulation that suggest an additional influence on decadal MOC variability having a Southern Hemisphere origin: dynamic signals originating in the Agulhas leakage region at the southern tip of Africa. These contribute a MOC signal in the tropical and subtropical North Atlantic that is of the same order of magnitude as the northern source. A complete rationalization of observed MOC changes therefore also requires consideration of signals arriving from the south.

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Figure 1: Strength of the interhemispheric transport in the Atlantic Ocean.
Figure 2: Mid-depth circulation in the high-resolution Agulhas nest.
Figure 3: Low-pass-filtered Agulhas-induced MOC anomalies.
Figure 4: Illustration of the wave processes conveying Agulhas-induced anomalies in the upper limb of the MOC.
Figure 5: Attribution of interannual MOC variability to different mechanisms.

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Acknowledgements

The integration of the experiments was performed at the Höchstleistungsrechenzentrum Stuttgart and the Computing Centre at Kiel University. We thank the NEMO and AGRIF System Teams as well as J.-M. Molines and M. Scheinert for technical support. The analysis was performed under the DFG project no. BO 907/2-2. J.R.E.L. received support from the Alexander von Humboldt-Stiftung.

Author Contributions A.B. and C.W.B. designed the experiments. A.B. implemented and conducted the experiments and carried out the 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. & Lutjeharms, J. Agulhas leakage dynamics affects decadal variability in Atlantic overturning circulation. Nature 456, 489–492 (2008) doi:10.1038/nature07426

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