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Impact of intensified Indian Ocean winds on mesoscale variability in the Agulhas system

Abstract

South of Africa, the Agulhas Current retroflects and a portion of its waters flows into the South Atlantic Ocean1, typically in the form of Agulhas rings2. This flux of warm and salty water from the Indian to the Atlantic Ocean (the Agulhas leakage) is now recognized as a key element in global climate3. An Agulhas leakage shutdown has been associated with extreme glacial periods4, whereas a vigorous increase has preceded shifts towards interglacials5. In the absence of a coherent observing system, studies of the Agulhas have relied heavily on ocean models, which have revealed a possible recent increase in Agulhas leakage6,7,8. However, owing to the high levels of oceanic turbulence, model solutions of the region are highly sensitive to their numerical choices9,10, stressing the need for observations to confirm these important model results. Here, using satellite altimetry observations from 1993 to 2009, we show that the mesoscale variability of the Agulhas system, in particular in the Mozambique Channel and south of Madagascar, has intensified. This seems to result from an increased South Equatorial Current driven by enhanced trade winds over the tropical Indian Ocean. Overall, the intensified mesoscale variability of the Agulhas system is reflected in accelerated eddy propagation, in its source regions as well as the retroflection from which eddies propagate into the South Atlantic Ocean. This suggests that the Agulhas leakage may have increased from 1993 to 2009, confirming previous modelling studies that have further implied an increased Agulhas leakage may compensate a deceleration of meridional overturning circulation associated with a freshening of the North Atlantic Ocean6,11.

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Figure 1: Wind-driven intensification of the subtropical Indian Ocean gyre.
Figure 2: Decadal trends of the surface wind stress of the subtropical Indian Ocean.
Figure 3: Intensification of MKE and EKE.

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Acknowledgements

This work has been supported by the Nansen-Tutu Centre for Marine Environmental Research and the International Centre for Education, Marine and Atmospheric Sciences over Africa. M.R. thanks the African Centre for Earth System Science, Water Research Commission and the National Research Foundation for financial support. The altimeter products were produced by Ssalto/Duacs and distributed by Aviso, with support from Cnes (www.aviso.oceanobs.com/duacs/). The CFSR data were obtained from the Research Data Archive of the Computational and Information Systems Laboratory at the National Center for Atmospheric Research. We would like to thank B. Pohl for his advice on the Antarctic Oscillation.

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B.C.B. initiated the study and together with P.P. collected and analysed the data. All authors interpreted the results and contributed towards writing the manuscript.

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Correspondence to Björn C. Backeberg.

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Backeberg, B., Penven, P. & Rouault, M. Impact of intensified Indian Ocean winds on mesoscale variability in the Agulhas system. Nature Clim Change 2, 608–612 (2012). https://doi.org/10.1038/nclimate1587

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