The oceanic overturning circulation has a central role in the Earth’s climate system and in biogeochemical cycling1,2, as it transports heat, carbon and nutrients around the globe and regulates their storage in the deep ocean. Mixing processes in the Antarctic Circumpolar Current are key to this circulation, because they control the rate at which water sinking at high latitudes returns to the surface in the Southern Ocean3,4,5,6,7,8. Yet estimates of the rates of these processes and of the upwelling that they induce are poorly constrained by observations. Here we take advantage of a natural tracer-release experiment—an injection of mantle helium from hydrothermal vents into the Circumpolar Current near Drake Passage9—to measure the rates of mixing and upwelling in the current’s intermediate layers over a sector that spans nearly one-tenth of its circumpolar path. Dispersion of the tracer reveals rapid upwelling along density surfaces and intense mixing across density surfaces, both occurring at rates that are an order of magnitude greater than rates implicit in models of the average Southern Ocean overturning4,5,6,7,8. These findings support the view that deep-water pathways along and across density surfaces intensify and intertwine as the Antarctic Circumpolar Current flows over complex ocean-floor topography, giving rise to a short circuit of the overturning circulation in these regions.
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A NERC Advanced Research Fellowship supported A.C.N.G. during the analysis and writing of this Letter. We gratefully acknowledge feedback from J. Ledwell and K. Polzin.
Author Contributions A.C.N.G. designed and conducted the analysis and wrote the letter. D.P.S. and A.J.W. discussed aspects of the methodology and results, and helped with the writing. W.R. provided many of the 3He data and advised on their use.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
This file contains Supplementary Notes containing a detailed description of the methodology, derivation of equations, and error analysis, Supplementary Figure 1 with Legend and additional references. (PDF 278 kb)
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Garabato, A., Stevens, D., Watson, A. et al. Short-circuiting of the overturning circulation in the Antarctic Circumpolar Current. Nature 447, 194–197 (2007). https://doi.org/10.1038/nature05832
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