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Interdecadal North-Atlantic meridional overturning circulation variability in EC-EARTH

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The Atlantic meridional overturning circulation (AMOC) in a 600 years pre-industrial run of the newly developed EC-EARTH model features marked interdecadal variability with a dominant time-scale of 50–60 years. An oscillation of approximately 2 Sverdrup (1 Sv = 106 m3 s−1) is identified, which manifests itself as a monopole causing the overturning to simultaneously strengthen (/weaken) and deepen (/shallow) as a whole. Eight years before the AMOC peaks, density in the Labrador-Irminger Sea region reaches a maximum, triggering deep water formation. This density change is caused by a counterclockwise advection of temperature and salinity anomalies at lower latitudes, which we relate to the north-south excursions of the subpolar-subtropical gyre boundary and variations in strength and position of the subpolar gyre and the North Atlantic Current. The AMOC fluctuations are not directly forced by the atmosphere, but occur in a delayed response of the ocean to forcing by the North Atlantic Oscillation, which initiates “intergyre”-gyre fluctuations. Associated with the AMOC is a 60-year sea surface temperature variability in the Atlantic, with a pattern and timescale showing similarities with the real-world Atlantic Multidecadal Variability. This good agreement with observations lends a certain degree of credibility that the mechanism that is described in this article could be seen as representative of the real climate system.

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The authors are grateful to Shuting Yang and Torben Schmidt of the Danish Meteorological Institute for performing the pre-industrial control run and providing us with the data. The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n212643 (THOR project). Geert Jan van Oldenborgh of KNMI is acknowledged for the fruitful discussions and his comments on a draft version of this article. We thank the editor and two anonymous referees for their constructive comments.

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Correspondence to Bert Wouters.

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This paper is a contribution to the special issue on EC-Earth, a global climate and earth system model based on the seasonal forecast system of the European Centre for Medium-Range Weather Forecasts, and developed by the international EC-Earth consortium. This special issue is coordinated by Wilco Hazeleger (chair of the EC-Earth consortium) and Richard Bintanja.

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Wouters, B., Drijfhout, S. & Hazeleger, W. Interdecadal North-Atlantic meridional overturning circulation variability in EC-EARTH. Clim Dyn 39, 2695–2712 (2012).

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