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Eddy-induced variability in Southern Ocean abyssal mixing on climatic timescales

Nature Geoscience volume 7, pages 577–582 (2014)Cite this article

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Abstract

The Southern Ocean plays a pivotal role in the global ocean circulation and climate1,2,3. There, the deep water masses of the world ocean upwell to the surface and subsequently sink to intermediate and abyssal depths, forming two overturning cells that exchange substantial quantities of heat and carbon with the atmosphere4,5. The sensitivity of the upper cell to climatic changes in forcing is relatively well established6. However, little is known about how the lower cell responds, and in particular whether small-scale mixing in the abyssal Southern Ocean, an important controlling process of the lower cell7,8, is influenced by atmospheric forcing. Here, we present observational evidence that relates changes in abyssal mixing to oceanic eddy variability on timescales of months to decades. Observational estimates of mixing rates, obtained along a repeat hydrographic transect across Drake Passage, are shown to be dependent on local oceanic eddy energy, derived from moored current meter and altimetric measurements. As the intensity of the regional eddy field is regulated by the Southern Hemisphere westerly winds9,10, our findings suggest that Southern Ocean abyssal mixing and overturning are sensitive to climatic perturbations in wind forcing.

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Figure 1: Location of the DIMES experiment, the SR1b section and the DIMES mooring array.
Figure 2: Current speed and turbulent dissipation rates across three repeat microstructure sections.
Figure 3: Relationship between abyssal turbulent dissipation and surface and abyssal kinetic energy anomaly at the DIMES mooring site.
Figure 4: Decadal variability in turbulent dissipation rate and EKE along the SR1b section in eastern Drake Passage.

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Acknowledgements

The DIMES experiment is supported by the Natural Environment Research Council (NERC) of the UK and the US National Science Foundation. K.L.S. is supported by NERC. We are grateful to J. Ledwell, A. Bogdanoff, P. Courtois, K. Decoteau, D. Evans and X. Liang for their help in data collection and acknowledge the valuable assistance and hard work of the crew and technicians on the RRS James Cook, the RRS James Clark Ross and the RV Thomas G. Thompson. We also thank A. Thompson who provided many helpful comments, and E. Murowinski, R. Lueck and F. Wolk from Rockland Scientific for their support in microstructure data analysis.

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Authors and Affiliations

  1. University of Southampton, National Oceanography Centre, Southampton SO14 3ZH, UK

    K. L. Sheen, A. C. Naveira Garabato, J. A. Brearley & A. Forryan

  2. British Antarctic Survey, Cambridge CB3 0ET, UK

    M. P. Meredith & J-B. Sallée

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

    M. P. Meredith

  4. Woods Hole Oceanographic Institution, 266 Woods Hole Road Woods Hole, Massachusetts 02543-1050, USA,

    K. L. Polzin, L. St. Laurent & J. M. Toole

  5. National Oceanography Centre, Southampton SO14 3ZH, UK

    D. A. Smeed & B. A. King

  6. Laboratoire d’Océanographie et du Climat, 75252 Paris, France

    J-B. Sallée

  7. Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA

    A. M. Thurnherr

  8. Climate Change Research Centre and ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney, New South Wales 2052, Australia

    S. N. Waterman

  9. University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada

    S. N. Waterman

  10. University of Exeter, EX4 4SB, UK

    A. J. Watson

Authors
  1. K. L. Sheen
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Contributions

K.L.S. conducted most of the data analysis. A.C.N.G. and K.L.S. conceived the idea for the work and wrote the manuscript. J.A.B. processed the mooring current velocity data and computed the mooring turbulent dissipation time series. K.L.S., J.A.B., A.C.N.G., A.F. and L.S.L. collected and processed microstructure data. B.A.K. was instrumental in collecting hydrographic data along the repeat SR1b section. A.M.T. collected and processed lowered-ADCP data for the three DIMES SR1b sections. A.C.N.G., J.M.T., D.A.S. and A.J.W. conceived and directed the DIMES project and along with J-B.S. planned research cruises. M.P.M., K.L.P., D.A.S., J-B.S. and S.N.W. gave analytical and conceptual advice throughout the project.

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Correspondence to K. L. Sheen.

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Sheen, K., Naveira Garabato, A., Brearley, J. et al. Eddy-induced variability in Southern Ocean abyssal mixing on climatic timescales. Nature Geosci 7, 577–582 (2014). https://doi.org/10.1038/ngeo2200

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