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

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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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Figure 1: Agulhas Current Time-series (ACT) instrumental array and mean Agulhas Current from April 2010 to February 2013.
Figure 2: Agulhas Current transport proxies based on regressions between transport and sea surface slope at each ACT mooring location.
Figure 3: Trends of sea level and oceanic transport across the Agulhas Current, showing broadening.
Figure 4: Kinetic energy analyses across the Agulhas Current, showing increased eddying.

Change history

  • 21 December 2016

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


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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 (

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Authors and Affiliations



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.

Corresponding author

Correspondence to Lisa M. Beal.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Spatiotemporal variance of oceanic transport and sea surface slope are similar across the Agulhas Current.

a, First four eigenmodes of the transport per unit distance from the ACT array. b, First four combined eigenmodes of sea surface height and slope, from Aviso along-track absolute dynamic topography (ADT) from satellite altimetry. Note that the ADT gradient is positive across most of the array, but is shown as negative for comparison with the southward Agulhas Current transports in a. Black dotted lines depict mooring positions.

Extended Data Figure 2 Agulhas Current transport proxies based on regressions of total transport with sea surface eigenmodes.

a, Proxy for jet or stream-wise transport Tjet. b, Proxy for boundary layer transport Tbox. The three years of in situ transports from the ACT array are shown as black lines. The trends of these two proxies are inconsistent, owing to observed broadening of the jet.

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Beal, L., Elipot, S. Broadening not strengthening of the Agulhas Current since the early 1990s. Nature 540, 570–573 (2016).

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