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Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release - Nature

  • ️Naveira Garabato, Alberto C.
  • ️Wed Sep 18 2013
  • Letter
  • Published: 18 September 2013

Nature volume 501, pages 408–411 (2013)Cite this article

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Abstract

Diapycnal mixing (across density surfaces) is an important process in the global ocean overturning circulation1,2,3. Mixing in the interior of most of the ocean, however, is thought to have a magnitude just one-tenth of that required to close the global circulation by the downward mixing of less dense waters4. Some of this deficit is made up by intense near-bottom mixing occurring in restricted ‘hot-spots’ associated with rough ocean-floor topography5,6, but it is not clear whether the waters at mid-depth, 1,000 to 3,000 metres, are returned to the surface by cross-density mixing or by along-density flows7. Here we show that diapycnal mixing of mid-depth (∼1,500 metres) waters undergoes a sustained 20-fold increase as the Antarctic Circumpolar Current flows through the Drake Passage, between the southern tip of South America and Antarctica. Our results are based on an open-ocean tracer release of trifluoromethyl sulphur pentafluoride. We ascribe the increased mixing to turbulence generated by the deep-reaching Antarctic Circumpolar Current as it flows over rough bottom topography in the Drake Passage. Scaled to the entire circumpolar current, the mixing we observe is compatible with there being a southern component to the global overturning in which about 20 sverdrups (1 Sv = 106 m3 s−1) upwell in the Southern Ocean, with cross-density mixing contributing a significant fraction (20 to 30 per cent) of this total, and the remainder upwelling along constant-density surfaces. The great majority of the diapycnal flux is the result of interaction with restricted regions of rough ocean-floor topography.

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Acknowledgements

We thank the officers and staff of the RV Thomas Thompson, RRS James Cook and RRS James Clark Ross for their assistance in making the observations at sea. We thank the UK Natural Environment Research Council and the US National Science Foundation for funding the DIMES experiment. A.J.W. thanks the Royal Society for support.

Author information

Author notes

  1. Andrew J. Watson, Marie-José Messias & Benjamin Mills

    Present address: Present address: College of Life and Environmental Sciences, University of Exeter, Laver Building, Exeter EX4 4QE, UK.,

Authors and Affiliations

  1. School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK,

    Andrew J. Watson, Marie-José Messias, Neill Mackay & Benjamin Mills

  2. Woods Hole Oceanographic Institution, Woods Hole, 02543, Massachusetts, USA

    James R. Ledwell

  3. National Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK,

    Brian A. King & Alberto C. Naveira Garabato

  4. British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK,

    Michael P. Meredith

  5. Scottish Association for Marine Science, Oban, PA37 1QA, UK,

    Michael P. Meredith

  6. University of Southampton, National Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK,

    Alberto C. Naveira Garabato

Authors

  1. Andrew J. Watson

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  2. James R. Ledwell

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  3. Marie-José Messias

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  4. Brian A. King

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  5. Neill Mackay

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  6. Michael P. Meredith

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  7. Benjamin Mills

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  8. Alberto C. Naveira Garabato

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Contributions

A.J.W., J.R.L., M.-J.M., M.P.M. and A.C.N.G. planned and directed the tracer experiment. M.J.M. led the chemical analysis at sea, and, together with A.J.W., J.R.L., N.M. and B.M., obtained the tracer data. A.J.W., J.R.L., M.P.M., N.M., B.A.K. and A.C.N.G. analysed the physical oceanographic data. N.M. carried out the two-dimensional model computations. M.P.M., A.J.W. and A.C.N.G. planned and directed the research cruises. A.J.W. wrote the initial draft of the paper and all authors contributed to its revision.

Corresponding author

Correspondence to Andrew J. Watson.

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Watson, A., Ledwell, J., Messias, MJ. et al. Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release. Nature 501, 408–411 (2013). https://doi.org/10.1038/nature12432

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  • Received: 14 March 2013

  • Accepted: 03 July 2013

  • Published: 18 September 2013

  • Issue Date: 19 September 2013

  • DOI: https://doi.org/10.1038/nature12432

Editorial Summary

Rough ocean floor drives vertical mixing in Southern Ocean

Mixing processes in the mid-depth ocean are poorly understood, making it unclear how these waters return to the surface or how abyssal water is transferred through mid-depths. Andrew Watson and colleagues carried out a large-scale ocean tracer release in the East Pacific Ocean in early 2009 and followed the tracer (targeted to a constant-density surface) for two years as it advected through the Drake Passage between South America and Antarctica. Their findings suggest that the rough bottom topography in the region is important for Southern Ocean mixing, increasing mixing across density gradients approximately 20-fold at a depth of 1,500 metres.