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Rapid freshening of the deep North Atlantic Ocean over the past four decades

Nature volume 416, pages 832–837 (2002)Cite this article

Abstract

The overflow and descent of cold, dense water from the sills of the Denmark Strait and the Faroe–Shetland channel into the North Atlantic Ocean is the principal means of ventilating the deep oceans, and is therefore a key element of the global thermohaline circulation. Most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse-gas concentrations predict a weakening thermohaline circulation in the North Atlantic as the subpolar seas become fresher and warmer1,2,3, and it is assumed that this signal will be transferred to the deep ocean by the two overflows. From observations it has not been possible to detect whether the ocean's overturning circulation is changing, but recent evidence suggests that the transport over the sills may be slackening4. Here we show, through the analysis of long hydrographic records, that the system of overflow and entrainment that ventilates the deep Atlantic has steadily changed over the past four decades. We find that these changes have already led to sustained and widespread freshening of the deep ocean.

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Figure 1: The origins of deep freshening in the Labrador Sea.
Figure 2: Evidence of sustained and rapid freshening throughout the system of overflow and entrainment that ventilates the deep Atlantic.
Figure 3: Maintenance of the freshening rate of Iceland-Scotland Overflow Water (ISOW) on its long spreading-path to the Labrador Sea.

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Acknowledgements

We thank S.-A. Malmberg and H. Valdimarsson for the use of unpublished hydro-data from the Icelandic Standard Sections Programme, and A. Clarke and J. Lazier, whose efforts provided much of the Labrador Sea data that we use here. This work was part of the data synthesis phase of the WOCE Program, of the VEINS Project of the MAST III Programme of the European Community, and of the NOAA Consortium on the Ocean's Role in Climate of Scripps Institution of Oceanography, San Diego and Lamont-Doherty Earth Observatory, Palisades, New York; it was supported in part by the Office of Polar Programs and Geosciences Directorate of the NSF.

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Authors and Affiliations

  1. Centre for Environment, Fisheries, and Aquaculture Science, Lowestoft, NR33 OHT, UK

    Bob Dickson & Stephen Dye

  2. Bedford Institute of Oceanography, Dartmouth, Nova Scotia, B2Y 4A2, Canada

    Igor Yashayaev

  3. Institut fur Meereskunde, 22529, Hamburg, Germany

    Jens Meincke & Juergen Holfort

  4. Marine Laboratory, PO Box 101, AB11 9DB, Aberdeen, UK

    Bill Turrell

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  1. Bob Dickson
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  4. Bill Turrell
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Correspondence to Bob Dickson.

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Dickson, B., Yashayaev, I., Meincke, J. et al. Rapid freshening of the deep North Atlantic Ocean over the past four decades. Nature 416, 832–837 (2002). https://doi.org/10.1038/416832a

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