Clim. Past Discuss., 6, 1267-1309, 2010
www.clim-past-discuss.net/6/1267/2010/
doi:10.5194/cpd-6-1267-2010
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This discussion paper has been under review for the journal Climate of the Past (CP). Please refer to the corresponding final paper in CP.
Variations of the Atlantic meridional overturning circulation in control and transient simulations of the last millennium
D. Hofer1,2, C. C. Raible1,2, and T. F. Stocker1,2
1Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland
2Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland

Abstract. The variability of the Atlantic meridional overturing circulation (AMOC) strength is investigated in control experiments and in transient simulations of up to the last millennium using the low-resolution Community Climate System Model version 3. In the transient simulations the AMOC exhibits enhanced low-frequency variability that is mainly caused by transitions between two semi-stable circulation states which amount to a 10 percent change of the maximum overturning. One transition is also found in a control experiment, but the time-varying external forcing significantly increases the probability of the occurrence of such events though not having a direct, linear impact on the AMOC. The transition from a high to a low AMOC state starts with a reduction of the convection in the Labrador and Irminger Seas and goes along with a changed barotropic circulation of both gyres in the North Atlantic and a gradual strengthening of the convection in the Greenland-Iceland-Norwegian (GIN) Seas. In contrast, the transition from a weak to a strong overturning is induced by decreased mixing in the GIN Seas. As a consequence of the transition, regional sea surface temperature (SST) anomalies are found in the midlatitude North Atlantic and in the convection regions with an amplitude of up to 3 K. The atmospheric response to the SST forcing associated with the transition indicates a significant impact on the Scandinavian surface air temperature (SAT) in the order of 1 K. Thus, the changes of the ocean circulation make a major contribution to the Scandinavian SAT variability in the last millennium.

Citation: Hofer, D., Raible, C. C., and Stocker, T. F.: Variations of the Atlantic meridional overturning circulation in control and transient simulations of the last millennium, Clim. Past Discuss., 6, 1267-1309, doi:10.5194/cpd-6-1267-2010, 2010.
 
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