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Earth System Dynamics An interactive open-access journal of the European Geosciences Union

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Earth Syst. Dynam., 5, 103-115, 2014
© Author(s) 2014. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
20 Feb 2014
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 timescales. Here we investigate multi-decadal variability of the Atlantic meridional overturning circulation (AMOC), Northern Hemisphere sea-ice extent and global mean temperature (GMT) in an ensemble of CMIP5 models under control conditions. We report an inter-annual GMT variability of about ±0.1° C originating solely from natural variability in the model ensemble. By decomposing the 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., 5, 103-115, doi:10.5194/esd-5-103-2014, 2014.
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