Model spread in the multidecadal variability of the winter North Atlantic Oscillation connected to stratosphere-troposphere coupling

A part of the uncertainties in global climate model projections over Europe arise from their underestimation of multidecadal variability in the winter-time North Atlantic Oscillation (NAO). This underestimation, however, remains poorly understood. Past studies have linked the weak multidecadal NAO variability in models to an underestimated atmospheric response to North Atlantic sea surface temperature variability. Using the CMIP6 large ensemble of climate models, we explore statistical relationships with physical drivers that may contribute to intermodel spread in NAO variability. We find a significant intermodel correlation between multidecadal NAO variability and multidecadal stratospheric polar vortex variability, as well as a stratosphere-troposphere coupling parameter that quantifies the relationship between stratospheric winds and the NAO. Models with the lowest NAO variance are associated with weaker polar vortex variability and a weaker stratosphere-troposphere coupling parameter. 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, although observational records are currently too short to provide a robust constraint on these indices.