Do CMIP5 Models Reproduce Observed Low-Frequency North Atlantic Jet Variability?

The magnitude of observed multidecadal variations in the North Atlantic Oscillation (NAO) in winter is at the upper end of the range simulated by climate models and a clear explanation for this remains elusive. Recent research shows that observed multidecadal winter NAO variability is more strongly associated with North Atlantic (NA) jet strength than latitude, thus motivating a comprehensive comparison of NA jet and NAO variability across the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Our results show that the observed peak in multidecadal jet strength variability is even more unusual than NAO variability when compared to the model-simulated range across 133 historical CMIP5 simulations. Some CMIP5 models appear capable of reproducing the observed low-frequency peak in jet strength, but there are too few simulations of each model to clearly identify which. It is also found that an observed strong multidecadal correlation between jet strength and NAO since the mid-nineteenth century may be specific to this period. Plain Language Summary The dominant pattern in sea level pressure variability over the North Atlantic (NA) is the North Atlantic Oscillation (NAO), which is strongly associated with climate variations over western Europe. However, major focus of current research is that climate models broadly do not reproduce large enough low-frequency (longer than 30years) NAO variability, with the implication that significant improvements in the skill of decadal weather prediction are possible. Motivated by this, we analyzed data from 44 of the world's leading models and delved more deeply into major underlying characteristics of low-frequency winter NAO variability, specifically (1) strength and (2) latitude of the belt of prevailing westerly winds that blow across the NA from the eastern United States to western Europe. Our study revealed that it is the strength of the westerlies, rather than latitude, that underlies models' difficulty in reproducing low-frequency NAO variability. This more detailed picture also really matters in terms of impacts on European climate, since a more positive NAO associated mainly with stronger westerlies has different climate impacts compared to one associated mainly with more poleward westerlies. This is emphasized by a further finding that either component of the westerlies can dominate NAO variability across different decades and even centuries.