| Summary: | Climate models systematically underestimate multidecadal variability of the North Atlantic Oscillation (NAO) compared to observations. It has been suggested that weak atmosphere-ocean coupling associated with Atlantic Multidecadal Variability may explain the weak NAO variance. However, it could also be a manifestation of the signal-to-noise paradox identified on seasonal-to-decadal timescales, which has been linked to weak eddy-mean flow feedback in models. Here we examine multi-model large ensemble simulations and show that around three fifths of the inter-model spread in multidecadal NAO variance can be explained by stratospheric polar vortex variability and stratosphere-troposphere coupling strength, and not by the magnitude of AMV-NAO coupling as previously suggested. We further show there is a relationship across models between multidecadal NAO variability and eddy-mean flow feedback strength, suggesting the mechanism for weak North Atlantic signals identified on seasonal to decadal timescales may also operate on multidecadal timescales.
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