Double jet streams and their connection to heatwaves in the Northern Hemisphere

Heat extremes are exacerbated by ongoing climate change and have severe consequences on humans and the environment. Climate change also leads to changes in atmospheric circulation that affect the jet stream. One configuration of the jet stream is the double jet, where the jet splits into two branches, potentially triggering persistent weather patterns and prolonged heat extremes. We conducted a comprehensive assessment of double jet stream states over the Northern Hemisphere and their connection to heatwaves in the extended summer period May to October for 1979 to 2023, using ERA5 data. The results show an increase in double jet frequency over North America, as well as in persistence over Asia and North America. More persistent double jets are associated with higher heatwave cumulative intensity. We identified Europe as a double jet stream hotspot region, with the most pronounced connection to heatwaves. 40–80% of heatwaves co-occur with double jet events in Europe, 30–60% in Asia, and 15–50% in North America, particularly in northern regions. Northern Europe, particularly areas north of 50°N, such as Scandinavia, the British Isles, the Baltic region, and western Russia, exhibit a pronounced and statistically significant connection between double jet stream occurrences and heatwaves. We also found a significant relationship between double jet events and heatwaves in some regions of Asia, particularly between 60°N to 80°N, as well as in central China. The pronounced connection in areas northward of 50°N broadly aligns with the position of the double jet stream wind minimum, where persistent weather conditions tend to prevail. Overall, our results reveal a significant connection between double jet events and land heat extremes in the Northern Hemisphere and the shift towards more persistent double jet events, underpinning their importance for extreme weather.