Princeton researchers investigate how climate change will affect the North Atlantic Oscillation–and thus European weather

Each winter, whether Hamburg is drenched or Barcelona is dry largely comes down to something called the North Atlantic Oscillation (NAO). This major weather pattern drives temperatures and precipitation across Europe, especially in winter—and it’s poised to change, due to rising CO₂.

The NAO—and thus, European winter weather—could undergo significant shifts as CO₂ levels increase in the atmosphere. A new paper published in Nature Partner Journal Climate and Atmospheric Science, led by Princeton University researchers, finds that future European winters may become warmer and wetter in the north and colder and drier in the south due to predicted changes in the NAO.

“We showed that the NAO will robustly become more positive in a warmer world with more CO₂,” said Ivan Mitevski, a Hess postdoctoral fellow at Princeton Geosciences and lead author of the study. “We also showed that the NAO’s variability will decrease—which, to our knowledge, no one has demonstrated before.”

The NAO is a measure of atmospheric pressure differences between the Azores and Iceland. When the NAO is positive, the pressure difference increases, which shifts the North Atlantic jet stream poleward and directs the storm track over northern Europe, bringing warmer temperatures and more rain to places such as Amsterdam and Copenhagen.

By contrast, during negative NAO events the pressure difference decreases, weakening the jet stream and shifting it over southern Europe, making places like Rome, Lisbon and Madrid wetter and warmer than average, while northern Europe becomes colder and drier.

Mitevski and Gabriel Vecchi—the Knox Taylor Professor of Geosciences at Princeton and director of Princeton’s High Meadows Environmental Institute—worked with colleagues to better understand how the NAO will behave under future climate scenarios fueled by increased CO₂. Previous studies suggested that the NAO may become more positive under climate change, but their results were generally statistically insignificant due to a combination of low signal-to-noise ratio in the North Atlantic region and the inclusion of other confounding radiative forcing agents, such as atmospheric aerosols, methane and ozone.

To strengthen their analysis, Miteveski and his colleagues focused exclusively on the effect of CO₂. They examined simulations from 60 climate models and conducted additional experiments at Princeton. To extract a clearer NAO signal from the background noise, they ran simulations in which CO₂ concentrations were doubled, quadrupled or octupled.

In their models, the researchers found that, as CO₂ levels rise, the researchers found that the NAO became more positive and less variable. They also found that extreme NAO positive events—those that are unusually strong and typically occur just once in a decade—are likely to become more common, while the frequency of extreme NAO negative events will decrease.

Uncertainty is inherent in predictive modeling, but as time passes and CO₂ levels continue to rise, the researchers expect a strong likelihood that the NAO will shift toward a more positive state. “These are idealized experiments,” Mitevski said. “But now we have higher confidence that the NAO becomes more positive when CO₂ concentrations increase, which would make northern European winters warmer and wetter.”