Abstract

As heatwaves and droughts are becoming more frequent and intense, such as in Western Europe, there is a growing interest in unraveling the physical mechanisms behind their occurrences and changes. Soil desiccation is critical for the self-intensification and self-propagation of heatwaves, but its relative importance compared to other well-known large-scale atmospheric mechanisms, such as persistent atmospheric blocking systems and horizontal warm advection, remains elusive, particularly in the context of a changing climate. Here we utilize machine learning along with intervention experiments to estimate the respective contributions of soil water content (C swc) and atmospheric circulation (C atm) to daily maximum temperature in Western Europe, with a particular focus on the 2022 summer events. Our results reveal that during the two unparalleled heatwave events that occurred in June and July of 2022, the impact of C swc on the heatwave intensity was comparable to C atm in continental dry-to-wet transition regions. Reviewing heatwaves in recent three decades, the percentage of heatwave areas that are significantly influenced by soil moisture-air temperature coupling has increased by 11.4% per decade. Additionally, about 21.7% areas, mostly in the transition zones, witness a significant increase in C swc ; while only 2.5% exhibit a substantial increase in C atm. Our study emphasizes the observation-based large and increasing importance of soil moisture coupling in intensifying summer heatwaves and provides insights into future climates in extra-tropical regions like Western Europe, where a warmer and drier future is anticipated. 10