Comparing the influence of Atlantic Multidecadal Variability and spring soil moisture on European summer heat waves

Abstract: In this work, we study and compare the influence of the Atlantic Multidecadal Variability (AMV) and of spring soil moisture in Southern Europe on the duration and intensity of European summer heat waves. We study common heat waves with return times of a few years like in previous studies, but we also propose a new methodological approach, return time maps, that allows us to study rare heat waves with return times from 10 to 50 years. We use the outputs from three climate models, namely IPSL-CM6A-LR, EC-Earth3, and CNRM-CM6-1, in which North Atlantic sea surface temperatures are restored towards the observed AMV anomalies. The three models give consistent results, with the exception of EC-Earth simulating a much greater influence of soil moisture. Typical AMV or spring soil moisture anomalies induce changes in the temperature and duration of heat waves that are of comparable amplitude, but follow different regional patterns. As might be expected, a positive AMV phase or low soil moisture induces hotter and longer typical heat waves over most of Europe. However, counter-intuitively, they also induce less heat wave days and cooler heat waves over part of Northeast Europe. For more extreme events, the influence of the AMV and soil moisture increase, according to rather similar regional patterns as for typical heat waves. However, while the amplitude of the influence is greater, the regions with decreased heat wave temperature and less heat wave days extend in size.