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Broadening not strengthening of the Agulhas Current since the early 1990s

Nature volume 540, pages 570573 (22 December 2016) | Download Citation

This article has been updated

Abstract

Western boundary currents—such as the Agulhas Current in the Indian Ocean—carry heat poleward, moderating Earth’s climate and fuelling the mid-latitude storm tracks1,2. They could exacerbate or mitigate warming and extreme weather events in the future, depending on their response to anthropogenic climate change. Climate models show an ongoing poleward expansion and intensification of the global wind systems, most robustly in the Southern Hemisphere3,4,5, and linear dynamical theory6,7 suggests that western boundary currents will intensify and shift poleward as a result3,8. Observational evidence of such changes comes from accelerated warming and air–sea heat flux rates within all western boundary currents, which are two or three times faster than global mean rates5,9,10. Here we show that, despite these expectations, the Agulhas Current has not intensified since the early 1990s. Instead, we find that it has broadened as a result of more eddy activity. Recent analyses of other western boundary currents—the Kuroshio and East Australia currents—hint at similar trends11,12,13,14,15. These results indicate that intensifying winds may be increasing the eddy kinetic energy of boundary currents, rather than their mean flow. This could act to decrease poleward heat transport and increase cross-frontal exchange of nutrients and pollutants between the coastal ocean and the deep ocean. Sustained in situ measurements are needed to properly understand the role of these current systems in a changing climate.

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Change history

  • 21 December 2016

    The long-term trends reported for Tbox and Tjet were switched.

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Acknowledgements

L.M.B. and S.E. acknowledge support from the US National Science Foundation, grant OCE-0850891. The altimeter products distributed by Aviso were produced by Ssalto/Duacs, with support from Cnes (http://www.aviso.altimetry.fr/duacs/).

Author information

Affiliations

  1. Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida, USA

    • Lisa M. Beal
    •  & Shane Elipot

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Contributions

L.M.B. designed, obtained funding for, and led the Agulhas Current Time-series experiment and wrote and edited this paper. S.E. performed the analyses and wrote the Methods section.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Lisa M. Beal.

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DOI

https://doi.org/10.1038/nature19853

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