Jet stream responsible for extreme weather

Jet streams are strong winds which occur at a height of approximately 10 kilometers and can reach speeds of up to 500 kilometers per hour. The polar front jet, a strong westerly wind in the temperate zone over the North Atlantic and Western Europe, is of particular importance for the weather in Europe. It steers moisture and weather systems from the Atlantic to Europe, which can lead to flood events. At the same time, shifts in this Atlantic-European jet stream can also lead to droughts in Central Europe. It is possible that climate change may change the properties of the jet and thus influence the occurrence of extreme weather events. Until recently, not enough data was available to be able to demonstrate this, however. The degree to which the jet stream actually fluctuates and how closely it is linked to weather extremes was therefore unknown.

To investigate this, a research group led by climatology professor Stefan Brönnimann from the Geographical Institute and the Oeschger Centre for Climate Research at the University of Bern have spent five years collecting, digitizing and evaluating tens of thousands of pieces of data from archives that they gathered dating from the years 1421 to 2023. The data includes early measurement series, historical documents (e.g., on the freezing dates of rivers) as well as natural climate archives such as tree rings and ice cores. In their study, which has recently been published in the journal Nature Geoscience, the research team shows that changes in the jet stream are largely random but nonetheless have far-reaching effects.

Random fluctuations dominate

“Droughts and floods like those we have experienced in recent years are rare,” explains Brönnimann, “so it is important to examine past events to find out how they relate to changes in the Atlantic-European jet stream.” It is particularly important to better understand the role of the jet stream in the development of extreme weather events, as droughts and floods have occurred more frequently in recent times and their extent is expected to increase further in the course of climate change.

The most important findings of the study concern the causes of these changes: although factors such as volcanic eruptions or the El Niño phenomenon have an influence, it is small. The shifts in the strength, geographical latitude and tilt of the jet stream primarily occur by chance – as a result of natural fluctuations in the atmospheric circulation.

Climate change has not yet had any impact on the jet stream

Some situations can occur frequently one after the other, however. “Certain extreme events such as floods only last a few days, but the weather conditions responsible for them repeat themselves and frequently shape an entire season,” says Stefan Brönnimann, “we can also see this in the extreme weather events that occurred in times gone by.”

Climate change has not yet affected the changes in the jet stream, though. “Although the fluctuations have been pronounced in recent times, they are within the range of changes that we were able to reconstruct for the last 600 years,” explains Brönnimann. The study, which is now available, is the key finding of a research project for which Stefan Brönnimann was awarded a prestigious “Advanced Grant” from the European Research Council (ERC).

Linking data with climate simulations

What is new is not only a better understanding of the jet stream, but also the methods used to gather the information. The jet was analyzed with a new, global three-dimensional climate reconstruction. It has a monthly resolution and is based on combining data with climate simulations. A simulation that obeys the laws of physics but has no knowledge of the actual weather is adjusted to the actual observations. Another new feature of the study is the amount of data used: this far exceeds the amounts used in other reconstructions.

The reconstructions can be used to better categorize possible future changes in the jet stream, as predicted by some climate models.