Did you know that one of the most significant atmospheric phenomena over the North Atlantic is increasingly having an impact on extreme weather in Europe? We're talking about North Atlantic Oscillation (NAO), a key factor influencing weather from heatwaves to floods. A new study sheds light on how climate change is altering the behavior of the summer NAO, which has important implications for attribution of extreme weather.

What is the North Atlantic Oscillation (NAO)?

The NAO is the dominant mode of atmospheric variability over the North Atlantic sector. It represents a large-scale a "rocking" pattern in the atmospheric pressure anomaly.

• Positive NAO phase reflects an enhanced south-north pressure gradient, associated with stronger zonal jet streams.
• Negative phase of the NAO on the contrary, it reflects a reduced pressure gradient, associated with weaker zonal jet winds.

Extreme summer NAO conditions have long been associated with severe summer weather events in Europe, such as heat waves, droughts and floodsThe increased likelihood of extreme summer weather in Europe over the past 40 years has raised concerns that global warming may affect the extremes of the summer NAO and thus increase the likelihood of heatwaves in Europe. However, previous studies have mainly focused on changes in the mean state of the NAO, not its variability or extreme states.

Climate change is expanding the variability of the summer NAO.

The latest findings show that the statistical distribution of the summer North Atlantic Oscillation index is expands with increasing global warmingThis means that the summer NAO shows increased variability due to global warming. This increased variability leads to higher probability of occurrence of extreme NAO states – for both positive and negative phases.

• Study defines positive extremes of the NAO as an index above 1.5 standard deviations and negative extremes below -1.5 standard deviations.
• Simulations with large ensembles of climate models (so-called “Large Ensembles” – LEs) and data reanalyses confirm this expanding the distribution of the summer NAO index.
• The increase in the occurrence of summer NAO extremes was consistent across different Earth system models.
• Indications of this increased probability also appear in historical periods, especially in 20th century data (20CR reanalysis).
• This increase is observable in most of the troposphere, from 500 hPa down to 200 hPa.

Enhanced impacts on surface temperatures in northwestern Europe

The increased frequency of extreme summer NAO conditions has direct consequences for climate. The study showed strengthening their impacts on surface temperature, especially in northwestern Europe.

• During positive extremes of the summer NAO with intensifies and extends the warming effect over northwestern Europe. These events also have a cooling effect over Greenland and the Mediterranean.
• During negative extremes of the summer NAO occurs increased cooling over northwestern EuropeIn addition, there is an increase in the warming effect over the Mediterranean.
• Analyses confirmed that these temperature dipole patterns are caused by summer NAO extremes, not the other way around.
• Although the models also show an amplification of the influence of summer NAO extremes on precipitation, this amplification was not statistically significant.

Causes of change: Dynamics of atmospheric currents

Changes in summer NAO extremes are associated with changes in atmospheric flow regimes. The study examined shifts in the position of the eddy-driven jet stream and the Greenland Blockade Index as indicators.

• With climate change, it is observed Poleward shift of wind flow and strengthening of the Greenland blockade.
• Increasing negative NAO extremes are associated with an increasing number of extreme Greenland Blockade episodes.
• Increasing positive NAO extremes are associated with an increasing number of extreme episodes of both northward wind deflection and Greenland Blockade.
• These findings suggest that in a transiently warming climate, both scenarios are likely to occur: "the faster they become even faster" (related to enhanced positive NAO extremes) and "the waves are getting wavy" (related to enhanced negative extremes of the NAO), rather than one dominating.

Implications and conclusion

The findings of this study are crucial because they highlight effects of climate change on the transient behavior of the atmosphereThey have important implications for attribution of extreme weather eventsAlthough the increase in the occurrence of NAO extremes appears more robust for the negative phase, even a small increase in positive extremes has a significant impact, as the average state of the summer NAO has a positive trend under global warming. Reanalysis data from the 20th century suggest that this increased probability of extreme summer NAOs has already appears in the historical period.

This shift in the dynamics of the summer North Atlantic Oscillation represents another layer of complexity that we must confront in a changing climate. Let us prepare for the possibility that summers in Europe may become increasingly extreme. Spring

A study published in Communications Earth & Environment magazine

Glossary of key terms

• North Atlantic Oscillation (NAO): The dominant regime of atmospheric variability over the North Atlantic sector, characterized by a large “wobble” pattern in atmospheric pressure anomalies between the Azores and Iceland.
• Positive phase of NAO: Combined with an enhanced south-north pressure gradient and stronger zonal jet streams, this leads to warmer and wetter weather in northern Europe and drier weather in southern Europe.
• Negative phase of NAO: Combined with a reduced pressure gradient and weaker zonal jet streams, this leads to cooler and drier weather in northern Europe and wetter weather in southern Europe.
• NAO Index: A quantitative measure of the state of the NAO, often derived from atmospheric pressure differences or empirical orthogonal function (EOF) analysis.
• NAO variability: The extent to which the NAO index changes over time. Increased variability means greater fluctuations and more frequent extremes.
• Extreme NAO conditions: The occurrence of an NAO index that differs significantly from the mean, usually defined as more than 1.5 standard deviations from the mean (for both positive and negative extreme events).
• Global warming: The long-term increase in the average temperature of the Earth's climate system, observed since the pre-industrial period, caused mainly by human activity, especially greenhouse gas emissions.
• Zonal jet stream: A band of strong, high-altitude westerly winds located in the upper troposphere. Its changes (shift, strengthening, weakening) are crucial for atmospheric circulation.
• Greenland blockade: The atmospheric blocking pattern occurring over Greenland disrupts the typical westerly airflow and can lead to persistent weather events in Europe.
• Empirical Orthogonal Function (EOF) Analysis: A statistical technique used to decompose spatial and temporal data into orthogonal modes helps identify dominant patterns of variability, such as the NAO.
• Large Ensembles (LEs): Ensembles of climate simulations that differ only slightly in initial conditions, allowing for the investigation of the internal variability of the climate system.
• Reanalysis data: Global, spatially continuous and temporally consistent data about the atmosphere and the Earth's surface, which are created by assimilating (combining) observations with a weather model. 20CR (20th Century Reanalysis) is one such dataset.
• RCP 8.5 (Representative Concentration Pathway 8.5): A scenario of future greenhouse gas concentrations that represents a high-emissions scenario (worst-case scenario) leading to significant global warming.
• Anthropogenic impact: Environmental impacts resulting from human activity, such as greenhouse gas emissions.
• Atmospheric flow regimes: Stable or semi-stable patterns of atmospheric circulation that can last for days to weeks and affect regional weather and climate.