Considerable yet contrasting regional imprint of circulation change on summer temperature trends across the Northern hemisphere mid-latitudes

Abstract. Rising summer temperatures and more frequent heat extremes are well-documented outcomes of anthropogenic climate change. However, the extent to which atmospheric circulation changes contribute to these trends remains contested. Regional differences across the northern mid-latitudes suggest that circulation plays a role, yet robustly quantifying its contribution over multiple decades is very challenging. We address this by systematically testing statistical and machine learning methods that decompose temperature signals into a thermodynamic and a dynamic contribution against climate model simulations. Specifically, we use unforced simulations with circulation nudged to match a forced simulation that includes anthropogenic emissions and land-use change. We apply decomposition methods to the forced simulations and compare their estimates of circulation-induced trends with those found in nudged circulation simulations. Our analysis reveals that most methods accurately identify the sign of circulation-induced changes in temperature, although they consistently underestimate their magnitude. Despite this limitation, the results demonstrate that circulation changes have made a substantial contribution to summer temperature trends across the northern mid-latitudes. In Europe, a hotspot region, we estimate that up to half of the observed summer warming between 1979 and 2023 can be attributed to circulation trends. Furthermore, circulation trends have contributed to warmer summer temperatures over Western North America, Central Siberia, Mongolia, Central China, and northeastern Canada. Yet, circulation changes have cooled summer temperatures over Eastern and Central North America, Eastern China, and Central Asia. Overall, our results, based on multiple methods, confirm a circumglobal mid-latitude pattern of considerable, yet contrasting, contributions of circulation changes to summer temperature trends.