External forcing of the North Atlantic Oscillation*

The North Atlantic Oscillation (NAO) is the dominant pattern of atmospheric circulation affecting western Europe and eastern North America. The observed NAO exhibits pronounced multidecadal variations but whether they are externally forced or internally generated is not understood. Furthermore, climate models appear to underestimate the likelihood of multidecadal NAO trends. Here we analyse multi-model simulations of the historical period since 1850. We find that multidecadal increases in the net energy imbalance at the top of the atmosphere warm the troposphere and create a thermal gradient at the latitude of the tropopause in the upper troposphere that shifts the jet and alters the NAO. In some models the tropopause is equatorward of the jet causing an equatorward shift while in other models and the real world it is poleward of the jet causing a poleward shift. This provides an emergent constraint and results in constrained simulations of multidecadal NAO changes that closely match observations, suggesting a key role for external forcing over the historical period. We further show that constrained projections suggest the NAO will increase to unprecedented levels under the business as usual scenario but this can be avoided with mitigation.