European heatwaves in present and future climate simulations: A Lagrangian analysis

Heatwaves are prolonged periods of anomolously high temperatures. Also in combination with droughts, heatwaves can have devastating impacts on the environment, society and economy.  Dynamically, heatwaves are linked to large-scale, quasi-stationary atmospheric blocking patterns. Heatwaves have become more intense and more numerous over most land areas in the recent history. Due to the expected rise in global mean temperature, it is expected that this trend will continue. This general intensification in line with mean temperature may be modified by changes also in the underlying dynamical processes.

In order to study such potential changes in heatwave dynamics, we compare Lagrangian backward trajectories of air streams associated with historic (1991-2000) and future (2091-2100) heatwaves in six European regions. The heatwaves are identified based on a large ensemble of CESM climate simulations with  the help of the percentile-based Heat Wave Magnitude Index daily.

Compared to climatological values, air streams associated with heatwaves show a larger temperature increase along their trajectory, which is connected to stronger descent and stronger diabatic heating when the air parcels enter the boundary layer. For future heatwaves, the model projects a north-/northeastward shift of the origin of the air masses three days prior to the heat event in most study regions. Furthermore, we find larger diabatic temperature increases along the parcel trajectories in the future. This increasing importance of diabatic heating is more pronounced for heatwaves over continental regions. Boundary-layer diabatic temperature changes are driven by sensible heat fluxes, which are stronger over dry soils, associated with an increase of the Bowen ratio. The amplified diabatic heating associated with future heatwaves may thus be explained by an expected drying of the land surface.