1Potsdam Institute for Climate Impact Research, Telegrafenberg A62, 14473 Potsdam, Germany
2Physics Institute, Potsdam University, Potsdam, Germany
3Department of Physics, Humboldt University, Berlin, Germany
Received: 07 Aug 2013 – Discussion started: 06 Sep 2013
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.
Revised: 15 Nov 2013 – Accepted: 05 Dec 2013 – Published: 20 Feb 2014
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.