Increase in persistence and intensity of heat waves in hot summers due to the intensification of the seasonal cycle

Summer seasons have warmed rapidly over western Europe increasing the number of hot days and severe heat waves. Notably, heat extremes have intensified more than the seasonal average temperatures as a result of the widening of daily temperature distributions and the heterogeneous warming of the season with the already warmer late summer warming more than early summer.Here we show that the above described changes are even more pronounced for extremely warm seasons amplifying their heat impacts. We compare summer seasons with return periods of 100 years in current climate to similarly rare seasons in pre-industrial climate. We obtain a sufficient number of simulated hot seasons by applying a rare event algorithm that efficiently simulates a hot summer ensemble by iteratively and systematically discontinuing ensemble members that are cold and cloning trajectories that are warm. We use the fully coupled Community Earth System Model (CESM2).In pre-industrial climate, warm weather in early summer is required to dry out soils allowing temperatures to rise in the second half of summer. In current climate, soils tend to be drier in summer and therefore the requirement of warm weather in early summer is less strict. As a result, in current climate 100-year summers, the accumulation of heat in late summer is more pronounced leading to longer lasting and more intense heat waves. We also show that this difference in the heat characteristics of similarly rare summer seasons goes beyond the climatological widening of the daily temperature distribution and the change in the seasonal cycle.