The jet stream – a “wind highway” that runs around the globe in mid-latitudes – has a significant impact on our weather because it determines the route and speed of the high and low pressure areas in our latitudes. At the same time, it is commonly assumed that the jet stream will be influenced by climate change - it will become wavier and slower. A study has now analyzed the extent to which the jet stream has actually changed over the last 40 years. The results contradict common theories because the wave movement of the wind band has not slowed down significantly. However, its fluctuations show seasonal fluctuations and regional peculiarities.
The jet stream, which travels around the globe at an altitude of eight to ten kilometers, is caused by pressure and temperature differences between different regions of our planet. Since the air near the equator is warmer and less dense than at the poles, strong winds arise in the upper region of the troposphere, which transport warm air poleward. Due to the Earth's rotation, they are deflected to the east and flow around the Earth with wind speeds of up to 500 kilometers per hour. Since the jet stream regulates the distribution of air pressure, it has a major influence on the weather. For example, a stagnant or particularly slow and strongly swinging pendulum movement of the jet stream promotes heat waves or persistent periods of heavy rain over Europe, for example.
Theories put to the test
Common hypotheses suggest that human-caused climate change could cause the jet stream to slow as warming polar regions weaken its driving force. It is assumed that the wave amplitudes of the wind band could also increase due to the changed atmospheric circulation. “Many theories make assumptions about what can be expected for the jet stream in the future - but they are all based on very idealized assumptions,” says Georgios Fragkoulidis from Johannes Gutenberg University in Mainz. “While the CO₂ input has a direct impact on global warming, there are chaotic processes in atmospheric circulation.”
In order to find out to what extent the jet stream has actually changed so far, Fragkoulidis analyzed the development of the jet stream between 1979 and 2019 for his study. He used data from the European Center for Medium-Range Weather Forecasts (ECMWF) and NASA. Fragkoulidis focused primarily on two important properties of the jet stream: How wavy is it and at what speed does it flow around the Earth? “If the jet stream is very wavy, cold air can flow into Germany from the north or hot air from the south - the probability of heat or cold waves increases,” explains the researcher. A slow jet stream is associated with prolonged periods of rain, heat or drought.
No slowdown so far
The result: “Many areas in the Northern Hemisphere experience a positive trend in wave amplitude in winter, but a negative trend in summer. In other words: the jet stream becomes wavier in winter and less wavy in summer.” This contradicts common theories, according to which the pendulum oscillations of the jet stream do not change depending on the season. Fragkoulidis also noted that changes in the jet stream vary from region to region. Different effects occur over North America than over China or Europe. “We cannot therefore give a simple answer along the lines of ‘The jet stream is becoming more or less wavy’; the matter is much more complex,” summarizes Fragkoulidis.
With regard to the speed at which the waves of the jet stream move eastwards, the current result contradicts previous assumptions about the slowing down of the jet stream: “Even if it may seem different: In the northern hemisphere, especially in Europe, the phase speed of the jet stream has increased Waves have not changed significantly in the last 40 years,” reports Fragkoulidis. In the southern hemisphere, however, the waves have actually accelerated significantly since 1979.
Basis for future research
The results form a basis for future forecasts on the further development of the jet stream in the face of climate change. “The patterns observed in this study suggest that future trends in upper tropospheric circulation will not necessarily be zonally symmetric or homogeneous,” Fragkoulidis writes. In further studies, he would also like to look for explanations for his findings and, for example, find out whether the fact that the jet stream is less wavy during the summer months helps to mitigate possible weather extremes.
Source: Georgios Fragkoulidis (Johannes Gutenberg University Mainz), Weather and Climate Dynamics, doi: 10.5194/wcd-3-1381-2022