Slowing of the Atlantic meridional overturning circulation at 25° N

Abstract

The Atlantic meridional overturning circulation carries warm upper waters into far-northern latitudes and returns cold deep waters southward across the Equator1. Its heat transport makes a substantial contribution to the moderate climate of maritime and continental Europe, and any slowdown in the overturning circulation would have profound implications for climate change. A transatlantic section along latitude 25° N has been used as a baseline for estimating the overturning circulation and associated heat transport2,3,4. Here we analyse a new 25° N transatlantic section and compare it with four previous sections taken over the past five decades. The comparison suggests that the Atlantic meridional overturning circulation has slowed by about 30 per cent between 1957 and 2004. Whereas the northward transport in the Gulf Stream across 25° N has remained nearly constant, the slowing is evident both in a 50 per cent larger southward-moving mid-ocean recirculation of thermocline waters, and also in a 50 per cent decrease in the southward transport of lower North Atlantic Deep Water between 3,000 and 5,000 m in depth. In 2004, more of the northward Gulf Stream flow was recirculating back southward in the thermocline within the subtropical gyre, and less was returning southward at depth.

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Figure 1: Station positions for transatlantic hydrographic sections taken in 1957, 1981, 1992, 1998 and 2004.
Figure 2: Vertical distribution of mid-ocean meridional geostrophic flow across 25° N section.

References

  1. 1

    Bryden, H. L. & Imawaki, S. in Ocean Circulation and Climate (eds Siedler, G., Church, J. & Gould, J.) Ch. 6.2 455–474 (Academic, London, 2001)

    Google Scholar 

  2. 2

    Hall, M. M. & Bryden, H. L. Direct estimates and mechanisms of ocean heat transport. Deep-Sea Res. 29, 339–359 (1982)

    Article  ADS  Google Scholar 

  3. 3

    Roemmich, D. & Wunsch, C. Two transatlantic sections: meridional circulation and heat flux in the subtropical North Atlantic Ocean. Deep-Sea Res. 32, 619–664 (1985)

    Article  ADS  Google Scholar 

  4. 4

    Lavín, A., Bryden, H. L. & Parrilla, G. Meridional transport and heat flux variations in the subtropical North Atlantic. Glob. Atmos. Ocean Syst. 6, 269–293 (1998)

    Google Scholar 

  5. 5

    Cubasch, U. et al. in Climate Change 2001: The Scientific Basis (ed. Houghton, J. T.) Ch. 9, 525–582 (Cambridge Univ. Press, Cambridge, UK, 2001)

    Google Scholar 

  6. 6

    Vellinga, M. & Wood, R. A. Global climatic impacts of a collapse of the Atlantic thermohaline circulation. Clim. Change 54, 251–267 (2002)

    Article  Google Scholar 

  7. 7

    Srokosz, M. New experiment deploys observing array in N. Atlantic to investigate rapid climate change. Eos 85(8), 78–83 (2004)

    Article  ADS  Google Scholar 

  8. 8

    Cunningham, S. A. RRS Discovery Cruise 279 (04 Apr–10 May 2004): A Transatlantic Hydrography Section at 24.5° N Cruise Report 54, 1–199 (Southampton Oceanography Centre, Southampton, 2005) http://eprints.soton.ac.uk/17527/

    Google Scholar 

  9. 9

    Fuglister, F. C. Atlantic Ocean Atlas of Temperature and Salinity Profiles and Data from the International Geophysical Year of 1957–1958. Woods Hole Oceanographic Institution Atlas Series 1, 1–209 (WHOI, Woods Hole, Massachusetts, 1960)

    Google Scholar 

  10. 10

    Parrilla, G., Lavín, A., Bryden, H., Garcia, M. & Millard, R. Rising temperatures in the subtropical North Atlantic Ocean over the past 35 years. Nature 369, 48–51 (1994)

    Article  ADS  Google Scholar 

  11. 11

    Baringer, M. O'N. & Molinari, R. Atlantic Ocean baroclinic heat flux at 24 to 26° N. Geophys. Res. Lett. 26, 353–356 (1999)

    Article  ADS  Google Scholar 

  12. 12

    Baringer, M. O'N. & Larsen, J. C. Sixteen years of Florida Current transport at 27° N. Geophys. Res. Lett. 28, 3179–3182 (2001)

    Article  ADS  Google Scholar 

  13. 13

    Bryden, H. L., Johns, W. E. & Saunders, P. M. Deep western boundary current east of Abaco: Mean structure and transport. J. Mar. Res. 63, 35–57 (2005)

    Article  Google Scholar 

  14. 14

    Gill, A. E. & Niiler, P. P. The theory of the seasonal variability in the ocean. Deep-Sea Res. 20, 141–177 (1973)

    Google Scholar 

  15. 15

    Jayne, S. R. & Marotzke, J. The dynamics of ocean heat transport variability. Rev. Geophys. 39, 385–411 (2001)

    Article  ADS  Google Scholar 

  16. 16

    Larsen, J. C. Transport and heat flux of the Florida Current at 27° N derived from cross-stream voltages and profiling data: theory and observations. Phil. Trans. R. Soc. Lond. A 338, 169–236 (1992)

    Article  ADS  Google Scholar 

  17. 17

    Schmitz, W. J. Jr & Richardson, W. S. On the transport of the Florida current. Deep-Sea Res. 15, 679–693 (1968)

    Google Scholar 

  18. 18

    Richardson, W. S., Schmitz, W. J. Jr & Niiler, P. P. The velocity structure of the Florida Current from the Straits of Florida to Cape Fear. Deep-Sea Res. 16 (suppl.), 225–231 (1969)

    Google Scholar 

  19. 19

    Niiler, P. P. & Richardson, W. S. Seasonal variability of the Florida Current. J. Mar. Res. 31, 144–167 (1973)

    Google Scholar 

  20. 20

    Meinen, C. S., Baringer, M. O. & Garcia, R. Florida Current Transport http://www.aoml.noaa.gov/phod/floridacurrent (NOAA/AOML, Miami, Florida, 2005)

    Google Scholar 

  21. 21

    Josey, S. & Grist, J. The NOC (formerly SOC) Air-Sea Flux Climatology http://www.noc.soton.ac.uk/JRD/MET/fluxclimatology.php (National Oceanography Centre, Southampton, 2005)

    Google Scholar 

  22. 22

    Woodruff, S. NCEP Real-time Marine Data http://www.cdc.noaa.gov/cdc/data.nmc.marine.html (NOAA/Climate Diagnostics Center, Boulder, Colorado, 2005)

    Google Scholar 

  23. 23

    Wijffels, S. E., Schmitt, R. W., Bryden, H. L. & Stigebrandt, A. Transport of freshwater by the oceans. J. Phys. Oceanogr. 22, 155–162 (1992)

    Article  ADS  Google Scholar 

  24. 24

    Trenberth, K. E., Large, W. G. & Olson, J. G. The mean annual cycle in global ocean wind stress. J. Phys. Oceanogr. 20, 1742–1760 (1990)

    Article  ADS  Google Scholar 

  25. 25

    Leaman, K. D. et al. Transport, potential vorticity, and current/temperature structure across Northwest Providence and Santaren Channels and the Florida Current off Cay Sal Bank. J. Geophys. Res. 100, 8561–8569 (1995)

    Article  ADS  Google Scholar 

  26. 26

    Ganachaud, A. Error budget of inverse box models: The North Atlantic. J. Atmos. Ocean. Technol. 20, 1641–1655 (2003)

    Article  ADS  Google Scholar 

  27. 27

    Häkkinen, S. & Rhines, P. B. Decline of subpolar North Atlantic circulation during the 1990s. Science 304, 555–559 (2004)

    Article  ADS  Google Scholar 

  28. 28

    Østerhus, S. & Gammelsrod, T. The abyss of the Nordic Seas is warming. J. Clim. 12, 3297–3304 (1999)

    Article  ADS  Google Scholar 

  29. 29

    Dickson, B. et al. Rapid freshening of the deep North Atlantic Ocean over the past four decades. Nature 416, 832–837 (2002)

    CAS  Article  ADS  Google Scholar 

  30. 30

    Hansen, B., Turrell, W. R. & Østerhus, S. Decreasing outflow from the Nordic seas into the Atlantic Ocean through the Faroe Bank channel since 1950. Nature 411, 927–930 (2001)

    CAS  Article  ADS  Google Scholar 

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Acknowledgements

The 2004 transatlantic hydrographic section along 25° N was supported by the Natural Environment Research Council as part of the Core Strategic Research Programme ‘Ocean Variability and Climate’ at Southampton Oceanography Centre. Analysis of the five sections along 25° N was also supported by NERC as part of the Rapid Programme. Comments on an earlier draft by J. Hirschi, W. Johns, S. Josey, C. Meinen, G. Parrilla, P. Rhines, P. Saunders, J. Toole, P. Vélez and R. Wood led to substantial improvement. Author Contributions All authors contributed equally to this work.

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Correspondence to Harry L. Bryden.

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

Supplmentary Figure 1

Cumulative mid-ocean geostrophic transport for the 1957, 1981, 1992, 1998 and 2004 hydrographic sections along 25°N. (PDF 1962 kb)

Supplementary Figure 2

Cumulative mid-ocean geostrophic transport in the upper 1000 m for the 1957, 1981, 1992, 1998 and 2004 hydrographic sections along 25°N. (PDF 5827 kb)

Supplementary Figure 3

Accumulated meridional transport (Sv) above 1000 m depth as a function of zonal distance from the African coast for each of the 5 sections in 1957, 1981, 1992, 1998 and 2004. (PDF 3312 kb)

Supplementary Figure Legends

Text to accompany the above Supplementary Figures. (DOC 23 kb)

Supplementary Table 1

Mid-ocean meridional geostrophic water mass transports (Sv) across 26°N in potential temperature (θ) classes (DOC 32 kb)

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Bryden, H., Longworth, H. & Cunningham, S. Slowing of the Atlantic meridional overturning circulation at 25° N. Nature 438, 655–657 (2005). https://doi.org/10.1038/nature04385

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