Circulation dampened heat extremes intensification over the Midwest US and amplified over Western Europe

Abstract

Heat extremes have intensified globally in recent decades, but the Midwest United States (MUS) experiences a weak cooling trend while Western Europe (WEU) shows remarkable warming trends. We show that atmospheric circulation greatly amplify warming in WEU and weaken it in the MUS. Circulation-induced trends lead to ~1°C cooling in MUS since 1979, reversing the weak thermodynamic warming effects. While in WEU, they cause ~1°C warming, contributing to one third of observed trends. The observed circulation- and thermodynamic-induced trends are outside the multi-model ensemble range for the MUS and at the edge of the range for WEU. This explains why the total observed trends in these regions are at the very low and high end of the model range. To what extent the circulation-induced trends are forced or unforced remains a key open question to constrain future projections. If circulation-induced trends are forced, WEU may experience further intensified heat extremes, while if they result from unforced internal variability, WEU warming would slow and MUS may experience rapidly intensifying heat extremes in the coming decades.