Earth Syst. Dynam. Discuss., 4, 967-1003, 2013
© Author(s) 2013. This work is distributed
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This discussion paper has been under review for the journal Earth System Dynamics (ESD). Please refer to the corresponding final paper in ESD.
The role of the North Atlantic overturning and deep-ocean for multi-decadal global-mean-temperature variability
C. F. Schleussner1,2, J. Runge1,3, J. Lehmann1,2, and A. Levermann1,2
1Potsdam Institute for Climate Impact Research, Telegrafenberg A62, 14473 Potsdam, Germany
2Physics Institute, Potsdam University, Potsdam, Germany
3Department of Physics, Humboldt University, Berlin, Germany

Abstract. Earth's climate exhibits internal modes of variability on various time scales. Here we investigate multi-decadal variability of the Atlantic meridional overturning circulation (AMOC) in the control runs of an ensemble of CMIP5 models. By decomposing global-mean-temperature (GMT) variance into contributions of the AMOC and Northern Hemisphere sea-ice extent using a graph-theoretical statistical approach, we find the AMOC to contribute 8% to GMT variability in the ensemble mean. Our results highlight the importance of AMOC sea-ice feedbacks that explain 5% of the GMT variance, while the contribution solely related to the AMOC is found to be about 3%. As a consequence of multi-decadal AMOC variability, we report substantial variations in North Atlantic deep-ocean heat content with trends of up to 0.7 × 1022 J decade−1 that are of the order of observed changes over the last decade and consistent with the reduced GMT warming trend over this period. Although these temperature anomalies are largely density-compensated by salinity changes, we find a robust negative correlation between the AMOC and North Atlantic deep-ocean density with density lagging the AMOC by 5 to 11 yr in most models. While this would in principle allow for a self-sustained oscillatory behavior of the coupled AMOC–deep-ocean system, our results are inconclusive about the role of this feedback in the model ensemble.

Citation: Schleussner, C. F., Runge, J., Lehmann, J., and Levermann, A.: The role of the North Atlantic overturning and deep-ocean for multi-decadal global-mean-temperature variability, Earth Syst. Dynam. Discuss., 4, 967-1003, doi:10.5194/esdd-4-967-2013, 2013.
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