On the Relationship between Atmospheric Blocking and Heatwaves in Germany

Atmospheric blocking describes a quasi-stationary weather pattern in midlatitudes which is characterised by a disruption of the westerly flow. Within the blocking anticyclone, large-scale subsidence of air effects dissipation of clouds. Intensive solar radiation during summer leads to a positive radiation balance causing heatwaves which are intensified by adiabatic warming of subsiding air. In case of longer persistence of the anticyclone, dry episodes can extend to droughts which intensifies the heat due to reduced latent heat flux. In this study, we investigate the relationship between blocking and heatwaves in Germany in reanalyses and a subset of selected CMIP6-Simulations. Using the heatwave definition and heatwave magnitude index by Russo et al. (2014), we find that heatwaves are related to the occurrence of blocking in Central Europe. The probability of heatwave occurrence is round about the factor 10 higher with blocking than without blocking. To quantify the value of blocking as predictor for heatwaves, the Heidke Skill Score (HSS) is calculated. The HSS shows that blocking gives an additional value on predicting heatwaves. Strong heatwaves (with a magnitude > 1.5 following Russo) have even a higher relationship to blocking. However, the HSS gives lower values compared to the consideration of all heatwaves. This is related to the low number of days with strong heatwaves compared to the high numbers of blocking days leading to a high number of false alarms. Comparing the relationship of blocking and heatwaves between reanalyses and historical CMIP6-Simulations yields that the models can reproduce the results of the reanalyses in general for all heatwaves. For strong heatwaves, some models underestimate the influence of blocking. Furthermore, we show that no model is simulating such a strong heatwave as 2003 occurred in Central Europe.