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Mixing and convection in the Greenland Sea from a tracer-release experiment

Nature volume 401, pages 902–904 (1999)Cite this article

Abstract

Convective vertical mixing in restricted areas of the subpolar oceans, such as the Greenland Sea, is thought to be the process responsible for forming much of the dense water of the ocean interior1,2. Deep-water formation varies substantially on annual and decadal timescales3,4,5, and responds to regional climate signals such as the North Atlantic Oscillation6,7; its variations may therefore give early warning of changes in the thermohaline circulation that may accompany climate change8. Here we report direct measurements of vertical mixing, by convection and by turbulence, from a sulphur hexafluoride tracer-release experiment in the central Greenland Sea gyre. In summer, we found rapid turbulent vertical mixing of about 1.1 cm2 s-1. In the following late winter, part of the water column was mixed more vigorously by convection, indicated by the rising and vertical redistribution of the tracer patch in the centre of the gyre. At the same time, mixing outside the gyre centre was only slightly greater than in summer. The results suggest that about 10% of the water in the gyre centre was vertically transported in convective plumes, which reached from the surface to, at their deepest, 1,200–1,400 m. Convection was limited to a very restricted area, however, and smaller volumes of water were transported to depth than previously estimated9. Our results imply that it may be the rapid year-round turbulent mixing, rather than convection, that dominates vertical mixing in the region as a whole.

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Figure 1: The location of the experiment.
Figure 2: Mean profiles of tracer concentration against depth.
Figure 3: Diagram of the mixing calculation and results.

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Acknowledgements

We thank the staff of RV Håkon Mosby, RV Johann Hjort and RRS James Clark Ross for their support, and B. Guest, S. C. Sutherland, M. I. Liddicoat, R. D. Ling and T. Fileman for assistance. The main support for this work was from the EU MAST-III programme, European sub-polar ocean project, phase 2. Additional support from the following national agencies was also important: NERC (UK) NRC (Norway) and NSF (USA).

Author information

Authors and Affiliations

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

    A. J. Watson, M.-J. Messias, K. I. C. Oliver, F. Carse & E. Guilyardi

  2. Department of Analytical and Marine Chemistry, Göteborg University, Göteborg, S-41296, Sweden

    E. Fogelqvist, T. Tanhua & K. A. Olsson

  3. Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, 1225 West Dayton Street, Madison, 53706, Wisconsin, USA

    K. A. Van Scoy, D. J. Cooper & J. A. Kruepke

  4. Department of Geology, University of Bergen, Allégaten 71, Bergen, N-5007, Norway

    T. Johannessen & E. Jansen

  5. School of Mathematics, University of East Anglia, Norwich , NR4 7TJ, UK

    D. P. Stevens

  6. Department of Marine Environment, Institute of Marine Research, Postboks 1870-Nordnes, Bergen, N-5024, Norway

    F. Rey

  7. The University of the Faroe Islands , Noatun, Tórshavn, FO-100, Faroe Islands

    K. Simonsen

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

    J. R. Ledwell

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  1. A. J. Watson
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Corresponding author

Correspondence to A. J. Watson.

Supplementary information

Averaging of property profiles along potential density surfaces

In reducing the data from the surveys of the Greenland Sea we used the procedure of averaging properties along surfaces of constant potential density. This corrects for vertical displacements due to internal waves or eddies which cause isopycnals to have a significant but temporary slope. Figure 1, shows the four average tracer profiles for the surveys discussed in the paper, where the ordinate is potential density reduced to a 500dB surface (s0.5). However, in order to derive "conventional" coefficients of turbulent exchange, which are expressed in units of diffusivity (length2 time-1) it is necessary to render the resulting average profiles back into units of depth. We adopted the procedure described by Ledwell and Watson (1991), in which average pressure-vs-density profiles from the surveys were employed to transform the density ordinate back into depth. Figure 2 shows the four potential density-vs-depth profiles used for these transformations.

The resulting average profiles of tracer are slightly less spread with depth if isopycnal averaging is used than if straightforward isobaric averaging was used. The isopycnal procedure allows us to plot the tracer results of one survey with the depth-vs-density curves of another, in order to investigate how much of the change in shape of the tracer profile with time is due to diapycnal mixing, and how much is due to the evolving depth-density structure. The SF6-vs-potential density curves of Figure 1 show that the tracer did not depart greatly from the density at which it was released during periods and regions not subject to convection (i.e survey 1, 2, and 3 outside the central gyre). The peak of the curve remains centered close to the density of release, though this density sinks in the water column with time. In contrast, the peak concentration of the tracer in the gyre centre moves to higher densities in response to convection. However, it also rises substantially in the water column, as figure 2 of the paper shows. The whole water column in this region becomes noticeably more dense and less stratified as a result of the convection, and the release isopycnal is displaced substantially towards the surface.

41586_1999_BF44807_MOESM1_ESM.gif

Supplementary Figure 1: Averaged SF6 tracer versus potential density (s0.5) for the surveys and regions described in the paper (GIF 18 kb)

41586_1999_BF44807_MOESM2_ESM.gif

Supplementary Figure 2: The average potential density versus depth curves obtained by averaging the same profiles as in figure 1 (GIF 18 kb)

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Watson, A., Messias, MJ., Fogelqvist, E. et al. Mixing and convection in the Greenland Sea from a tracer-release experiment . Nature 401, 902–904 (1999). https://doi.org/10.1038/44807

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