Cold-season atmospheric response to the natural variability of the Atlantic meridional overturning circulation
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The influence of the natural variability of the Atlantic meridional overturning circulation (AMOC) on the atmosphere is studied in multi-centennial simulations of six global climate models, using Maximum Covariance Analysis (MCA). In all models, a significant but weak influence of the AMOC changes is found during the Northern Hemisphere cold-season, when the ocean leads the atmosphere by a few years. Although the oceanic pattern slightly varies, an intensification of the AMOC is followed in all models by a weak sea level pressure response that resembles a negative phase of the North Atlantic Oscillation (NAO). The signal amplitude is typically 0.5 hPa and explains about 10% of the yearly variability of the NAO in all models. The atmospheric response seems to be due primarily due to an increase of the heat loss along the North Atlantic Current and the subpolar gyre, associated with an AMOC-driven warming. Sea-ice changes appear to be less important. The stronger heating is associated to a southward shift of the lower-tropospheric baroclinicity and a decrease of the eddy activity in the North Atlantic storm track, which is consistent with the equivalent barotropic perturbation resembling the negative phase of the NAO. This study thus provides some evidence of an atmospheric signature of the AMOC in the cold-season, which may have some implications for the decadal predictability of climate in the North Atlantic region.
KeywordsAtlantic meridional overturning circulation Air-sea interactions North Atlantic GCM Decadal variability
The research leading to these results has received funding from the European Community’s 7th framework programme (FP7/2007-2013) under grant agreement No. GA212643 (THOR: "Thermohaline Overturning—at Risk", 2008–2012). For CF, support from the Institut Universitaire de France is gratefully acknowledged. We also thank T. Joyce, J. Mignot, and two anonymous reviewers for their useful comments and suggestions.
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