Due to global warming, the North Atlantic Oscillation, an atmospheric circulation pattern that strongly influences European weather, is becoming more extreme in summer
To the point
- North Atlantic Oscillation (NAO): Fluctuations in the air pressure difference between the Azores and Iceland influence the weather in Europe
- Increasing variability: Climate change will lead to more frequent and more severe NAO extremes in summer by the end of the century, with implications for extreme weather events such as heat waves
- NAO extremes already more frequent today: Historical data show that summertime NAO extremes have already become more frequent since the beginning of the observational record
In recent decades, extreme events such as heatwaves and droughts have become more frequent in Europe. But sometimes a striking division within the continent occurs, as was seen in the summer of 2023: While it was quite rainy in Germany and northwestern Europe, devastating forest fires blazed in southern Europe amid exceptionally high temperatures. Such contrasting weather patterns in Europe are caused by a phenomenon known as the North Atlantic Oscillation (NAO). The NAO is characterized by fluctuations in the air pressure difference between the Azores and Iceland over the North Atlantic. These fluctuations strongly influence the weather in Europe, leading to contrasting weather conditions in different regions. In the summer of 2023, the NAO was in a strong negative phase, which is associated with a weak atmospheric pressure difference in the North Atlantic. This brought cool, moist air to northwestern Europe and warm air to the Mediterranean. During a positive phase of the summertime NAO, the opposite occurs, with heat in northwestern Europe and lower temperatures in southern Europe.
Difference between the average temperature of summertime NAO extremes (left for positive, right for negative) and the average summer temperature of the whole decade (2090-2099) in the Max Planck Institute Earth System Model (MPI-ESM) climate change simulations following the Representative Concentration Pathway 8.5 (RCP8.5), which describes a future with continuing, high emissions of greenhouse gases, resulting in significant climate change by 2100.
© Quan Liu, MPI-M
In a new study, researchers from the Max Planck Institute for Meteorology and the University of Hamburg have shown that such summertime NAO extremes, and thus extreme weather conditions in Europe, are becoming more likely as a result of global warming. Previous studies had primarily examined the wintertime NAO because the fluctuations are more pronounced in winter and thus gain more attention. Meanwhile, summer NAO fluctuations directly influence agriculture and human well-being.
Amplified variability, amplified temperatures
The new study led by Quan Liu, scientist at the Max Planck Institute for Meteorology, was dedicated to the changes in the occurrence of these strong positive and negative phases of the summer NAO and the associated effects on European weather extremes in the course of global warming. Since extremes occur very rarely by definition, a sufficiently large database is required to reliably determine their changes. Using several climate models and ensembles of up to one hundred simulations of the climate from 1850 to 2100, the scientists investigated NAO trends and changes in its variability in an up to four-degree warmer world.
Increase in the occurrence of positive (left) and negative (right) extremes of the summertime NAO in the MPI-ESM climate change simulations following RCP8.5 (brown curve) and an idealized scenario in which the carbon dioxide content of the atmosphere increases by 1% per year (red curve). The shading shows the 95% confidence interval.
© CC BY 4.0 Liu et al., 2025. DOI: 10.1038/s43247-025-02422-x
The results were robust: They revealed a positive trend in the mean of the summertime NAO, that is, a trend towards a stronger pressure difference between the Azores and Iceland. "But what's even more interesting is that the variability is increasing. That means that there will be more and stronger extremes of the summertime NAO - both positive and negative phases," explains Liu. The associated heatwaves would be further amplified by a stronger link between extreme states of atmospheric circulation associated with the NAO and temperatures under climate change conditions. "Increase in summertime NAO extremes can have devastating effects on the European economy and people's health," says co-author Daniela Matei.
The team also looked into the past: Using observation-based so-called reanalysis data, they investigated whether extreme NAO conditions have already become more frequent since the beginning of the observational record. And indeed, there have been more NAO extremes in the most recent 40 years compared to the time period of 1850-1889. Next the researchers plan to investigate the physical mechanisms behind the increasing variability of the NAO, as well as the role of another atmospheric phenomenon-the East Atlantic Pattern-in weather extremes in Europe.
