Physical mechanism of the rapid increase in intense and long-lived extreme heatwaves in the Northern Hemisphere since 1980

Since 1980, both the intensity and duration of summer heatwaves in the middle and high latitudes of the Northern Hemisphere have significantly increased, leading this region to become a critical area for a significant increase in the frequency of intense and long-lived extreme heatwaves. We found that stronger and more persistent high-pressure systems and lower soil moisture before the events were the main drivers of intense and long-lived extreme heatwaves in western Europe and the middle and high latitudes of North America. However, in eastern Europe and Siberia, lower cloud cover before events is also a main driver of this type of extreme heatwave, in addition to the above drivers. These factors are coupled with each other and can change heatwave intensity and duration by influencing surface radiation processes during events. Using the self-organizing map classification method, we found that 6 weather patterns with increased frequency, intensity, and duration were the main dynamic reasons leading to the increase in intense and long-lived extreme heatwaves after 1980. In addition, the decrease in summer average soil moisture in most areas of the mid-high latitudes of the Northern Hemisphere and the decrease in average cloud cover in eastern Europe and Siberia are found to be the main thermodynamic reasons leading to the increase in these extreme heatwaves.