The role of atmospheric circulation changes in Western European warm season heat extremes

Abstract. Climate change has led to an intensification of summer heat extremes, with especially pronounced warming over Western Europe. Here, the maximum and mean of the daily maximum summer temperatures have warmed 3.3 and 2.4 times faster than global mean temperatures. A large part of this enhanced warming can be attributed to dynamical changes. The effects of climate change on springtime heat extremes and circulation changes are less well understood, though changes in spring can influence summer via soil moisture memory. Here we show that between 1950 and 2023, the maximum and mean of the daily maximum spring temperatures in Western Europe have intensified 2.2 and 2.0 times faster than global warming respectively. We show that most of this enhanced warming can be attributed to thermodynamical effects. However, using circulation analogues, we show that locally more than a third of the total temperature trends can be attributed to changes in atmospheric circulation. Specifically, southerly flow patterns, characterized by high pressure over Western Europe and low pressure over the Eastern Atlantic, are found to become more frequent and intense in spring, contributing to the warming trend. Finally, individual ensemble members from large ensemble historical climate model simulations show that those models are capable of simulating temperature trends nearly as extreme as observed, but the model mean underestimates the Western European trends.