The Changing View on How Freshwater Impacts the Atlantic Meridional Overturning Circulation

  • Michael Vellinga
  • Bob Dickson
  • Ruth Curry

These days, it would be generally accepted that through its northward transport of warm tropical waters, the Atlantic Meridional Overturning Circulation (AMOC) contributes effectively to the anomalous warmth of northern Europe (Large and Nurser 2001; see also Rhines and Hakkinen 2003; Rhines et al., this volume). The oceanic fluxes of mass, heat and salt that pass north across the Greenland–Scotland Ridge from the Atlantic to the Arctic Mediterranean have now been soundly established by direct measurement under the EC VEINS and ASOF/MOEN programmes, as have the corresponding fluxes to the Arctic Ocean (Ingvaldsen et al. 2004a, b; Schauer et al. 2004). We now know that the 8.5 million cubic metres per second of warm salty Atlantic Water that passes north across this Ridge carries with it, on average, some 313 million megawatts of power and 303 million kilograms of salt per second (Østerhus et al. 2005). As it returns south across the Ridge in the form of the two dense overflows from Nordic Seas, its salinity has decreased from about 35.25 to 34.88 and its temperature has dropped from 8.5 °C to 2.0 °C or less. Not surprisingly, surrendering this amount of heat is of more than local climatic importance. To quantify its contribution to climate the AMOC was deliberately* shut down in the HadCM3 Atmosphere-Ocean General Circulation Model by artificially releasing a large pulse of freshwater in the northern North Atlantic (Wood et al. 2003; Vellinga 2004; Wood et al. 2006). The cooling of mean air temperature over the northern Norwegian Sea and Barents Sea in the first 10 years after shutdown exceeds −15 °C, and some lesser degree of cooling is evident over the entire Hemisphere. In addition, significant changes in rainfall are evident (especially at low latitudes, Vellinga and Wood 2002), as well as changes in sea level height (Levermann et al. 2005; Vellinga and Wood 2007). [*note that this is a ‘what if’ experiment. The response of the AMOC to more plausible scenarios of gradual anthropogenic greenhouse gas increase is discussed in Section 12.3.2 of this chapter.]


Atlantic Meridional Overturning Circulation North Atlantic Oscillation Thermohaline Circulation Freshwater Flux Atlantic Meridional Overturning Circulation Variability 
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Copyright information

© Springer Science + Business Media B.V 2008

Authors and Affiliations

  • Michael Vellinga
    • 1
  • Bob Dickson
    • 2
  • Ruth Curry
    • 3
  1. 1.Met Office Hadley CentreUK
  2. 2.Centre for Environment, Fisheries and Aquaculture ScienceUK
  3. 3.Woods Hole Oceanographic InstitutionWoods HoleUSA

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