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


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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  1. Killworth,P. D. Deep convection in the world ocean. Rev. Geophys. Space Phys. 21, 1–21 (1983).

    Article  ADS  Google Scholar 

  2. Rudels,B. The thermohaline circulation of the Arctic Ocean and the Greenland Sea. Phil. Trans. R. Soc. Lond. A352, 287– 299 (1995).

    Article  ADS  Google Scholar 

  3. Mosby,H. Deep Water in the Norwegian Sea. Geofis. Publ.21(3), 1–62 (1959).

    Google Scholar 

  4. Schlosser,P., Bönisch,G., Rhein,M. & Bayer,R. Reduction of deep-water formation in the Greenland Sea during the 1980s—evidence from tracer data. Science251, 1054– 1056 (1991).

    Article  ADS  CAS  Google Scholar 

  5. Meincke,J., Jonsson,J. & Swift,J. H. Variability of convective conditions in the Greenland Sea. Int. Council Explorat. Seas Mar. Sci. Symp.195 , 32–39 (1992).

    Google Scholar 

  6. Dickson,R., Lazier,J., Meincke,J., Rhines,P. & Swift,J. Long term co-ordinated changes in the convective activity of the North Atlantic. Prog. Oceanogr.38, 241– 295 (1996).

    Article  ADS  Google Scholar 

  7. Delworth,T. L., Manabe,S. & Stouffer, R. J. Multidecadal climate variability in the Greenland Sea and surrounding regions: A coupled model simulation. Geophys. Res. Lett.24, 257–260 ( 1997).

    Article  ADS  Google Scholar 

  8. Manabe,S. & Stouffer,R. J. Century-scale effects of increased atmospheric CO2 on the ocean–atmosphere system. Nature364, 215–218 ( 1993).

    Article  ADS  CAS  Google Scholar 

  9. Rhein,M. Convection in the Greenland Sea, 1982–1993. J. Geophys. Res. 101, 18183–18192 (1988).

    Article  ADS  Google Scholar 

  10. Watson,A. J. & Ledwell,J. R. Purposefully released tracers. Phil. Trans. R. Soc. Lond. A325, 189– 200 (1988).

    Article  ADS  CAS  Google Scholar 

  11. Ledwell,J. R. & Watson,A. J. The Santa-Monica Basin tracer experiment—a study of diapycnal and isopycnic mixing. J. Geophys. Res. 96, 8695–8718 (1991).

    Article  ADS  Google Scholar 

  12. Ledwell,J. R., Watson,A. J. & Law,C. S. Evidence for slow mixing across the pycnocline from an open-ocean tracer-release experiment. Nature364 , 701–703 (1993).

    Article  ADS  CAS  Google Scholar 

  13. Killworth,P. D. On “chimney” formations in the ocean. J. Phys. Oceangr. 9, 531–554 ( 1979).

    Article  ADS  Google Scholar 

  14. Ivey,G. N., Taylor,J. R. & Coates, M. J. Convectively driven mixed-layer growth in a rotating, stratified fluid. Deep Sea Res. I42, 331 –349 (1995).

    Article  Google Scholar 

  15. Visbeck,M., Fischer,J. & Schott,F. Preconditioning the Greenland Sea for deep convection—ice formation and ice drift. J. Geophys. Res.100, 18489–18502 (1995).

    Article  ADS  Google Scholar 

  16. Law,C. S., Watson,A. J. & Liddicoat, M. I. Automated vacuum analysis of sulfur-hexafluoride in seawater—derivation of the atmospheric trend (1970–1993) and potential as a transient tracer. Mar. Chem.48, 57–69 (1994).

    Article  CAS  Google Scholar 

  17. Ledwell,J. R., Watson,A. J. & Law,C. S. Mixing of a tracer in the pycnocline. J. Geophys. Res.103, 21499–22529 (1998).

    Article  ADS  Google Scholar 

  18. Bönisch,G., Blindheim,J., Bullister,J. L., Schlosser,P. & Wallace,D. W. R. Long-term trends of temperature, salinity, density, and transient tracers in the central Greenland Sea. J. Geophys. Res.102, 18553–18571 (1997).

    Article  ADS  Google Scholar 

  19. Mauritzen,C. Production of dense overflow waters feeding the North-Atlantic across the Greenland-Scotland Ridge. 1. Evidence for a revised circulation scheme. Deep Sea Res. I43, 769–806 (1996).

    Article  Google Scholar 

  20. Aagaard,K., Fahrbach,E., Meincke,J. & Swift,J. H. Saline outflow from the Arctic-Ocean—its contribution to the deep waters of the Greenland, Norwegian, and Iceland Seas. J. Geophys. Res.96, 20433–20441 (1991).

    Article  ADS  CAS  Google Scholar 

  21. Bevington,P. R. & Robinson,K. Data Reduction and Error Analysis for the Physical Sciences (McGraw-Hill, New York, 1992).

    Google Scholar 

  22. Klinger,B. A., Marshall,J. & Send,U. Representation of convective plumes by vertical adjustment. J. Geophys. Res.101, 18175– 18182 (1996).

    Article  ADS  Google Scholar 

  23. Send,U. & Marshall,J. Integral effects of deep convection. J. Phys. Oceangr.25, 855– 872 (1995).

    Article  ADS  Google Scholar 

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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).

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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.


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


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).

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