Model spread in multidecadal NAO variability connected to stratosphere-troposphere coupling

Abstract. The underestimation of multidecadal variability in the winter-time North Atlantic Oscillation (NAO) by global climate models remains poorly understood. Understanding the origins of this weak NAO variability is important for making model projections more reliable. Past studies have linked the weak multidecadal NAO variability in models to an underestimated atmospheric response to the Atlantic Multidecadal Variability (AMV). We investigate historical simulations from CMIP6 large ensemble models and find that most of the models do not reproduce observed multidecadal NAO variability, as found in previous generations of climate models. We explore statistical relationships with physical drivers that may contribute to intermodel spread in NAO variability. There is a significant anti-correlation across models between the AMV-NAO coupling parameter and multidecadal NAO variability over the full historical period (r=-0.55, p<0.05). However, this relationship is relatively weak and becomes obscured when using a common period (1900–2010) and detrending the data in a consistent way with observations to enable a model-data comparison. This suggests that the representation of NAO-AMV coupling contributes to a modest proportion of intermodel spread in multidecadal NAO variability, although the importance of this process for model spread could be underestimated given evidence of a systematically poor representation of the coupling in the models. We find a significant intermodel correlation between multidecadal NAO variability and multidecadal stratospheric polar vortex variability and a stratosphere-troposphere coupling parameter, which quantifies the relationship between stratospheric winds and the NAO. The models with the lowest NAO variance are associated with weaker polar vortex variability and a weaker stratosphere-troposphere coupling parameter. The two stratospheric indices are uncorrelated across models and together give a pooled R² with NAO variability of 0.7, which is larger than the fraction of intermodel spread related to the AMV (R²=0.3). The identification of this relationship suggests that modelled spread in multidecadal NAO variability has the potential to be reduced by improved knowledge of observed multidecadal stratospheric variability; however, observational records are currently too short to give a robust constraint on these indices.