Globally heat extremes have intensified in recent decades. However, while Western Europe shows a remarkably strong intensification of heat extremes, the Midwest United States experienced only weak warming of warmest nighttime and even a weak decrease in the intensity of daytime heat extremes since 1979. Here, we show that for daytime heat extremes in the Midwest United States atmospheric circulation induced -1°C cooling since 1979, reversing the thermodynamic warming trend. The observed circulation-induced trend is outside the multi-model range and the overall trend at the very low end of it. In Western Europe circulation greatly amplified warming by -1°C, accounting to one third of observed trend. The observed circulation and thermodynamic-induced trends, as well as the total observed trends are at the high end of the model range in Western Europe. Understanding whether the strong circulation-induced trends are externally forced or unforced internal variability remains key to constrain future trends in heat extremes.

Jitendra Singh, Sebastian Sippel, Erich M. Fischer