Observed intensification of heat extremes amplified by circulation over western Europe and dampened over the US Midwest

Heat extremes show the fastest warming trend over western Europe (WEU) and a weak cooling trend over the Midwest United States (MUS). We use observations and Earth System Model (ESM) large ensemble simulations to understand why the observed trends over these two regions are opposite. Based on the dynamical adjustment method we provide observational and model evidence that circulation changes greatly amplify the warming trends over WEU and weaken the trends over MUS in the last four decades. We find that circulation-driven changes in heat extremes cause ~0.2°C/decade cooling over MUS that reverses the weaker warming effects of all other forcings combined and thus leads to very small overall trends. In contrast, it causes an additional ~0.2°C/decade warming over WEU, which accounts for ~35% of the warming rate that is caused by forced thermodynamic changes. Although ESMs represent the forced thermodynamic warming well over WEU, they underestimate the circulation-induced warming that further reveals why ESMs underestimate the observed warming rate over WEU.

Overall, these findings imply that if the circulation changes represent a forced response WEU continues to experience a severe intensification of heat extremes conditions in future as circulation-induced amplification of heat extremes may further intensify in the warmer world. Conversely, if the circulation-induced trend were due to internal variability, and thus would reverse in the coming decades, this would imply a somewhat slower rate of warming of heat extremes. Moreover, heat extremes over MUS continue to warm more slowly if circulation keeps dampening the warming effects in the coming decades, but potentially increase rapidly if the circulation trend reverses.