Letter

Nature 447, 194-197 (10 May 2007) | doi:10.1038/nature05832; Received 21 December 2006; Accepted 12 April 2007

Short-circuiting of the overturning circulation in the Antarctic Circumpolar Current

Alberto C. Naveira Garabato1, David P. Stevens2, Andrew J. Watson3 & Wolfgang Roether4

  1. School of Ocean and Earth Science, National Oceanography Centre, Southampton SO14 3ZH, UK
  2. School of Mathematics, University of East Anglia, Norwich NR4 7TJ, UK
  3. School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
  4. Institut für Umweltphysik, Universität Bremen, Bremen D-28334, Germany

Correspondence to: Alberto C. Naveira Garabato1 Correspondence and requests for materials should be addressed to A.C.N.G. (Email: acng@noc.soton.ac.uk).

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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