Long vs. short: understanding the dynamics of persistent summer hot spells in Europe

The persistence of surface hot spells in Europe on subseasonal timescales can lead to significant socio-economic impacts. Here, we adopt a regional perspective to compare the dynamical features associated with long-lasting persistent (12–26 d) and short-lived (4–5 d) regional-scale hot spells in Europe during summer using the ERA5 reanalysis. We identify six coherent regions in Europe (Southwestern (SW) Europe, Western (W) Europe, Central-Southern Europe, Northern Europe, Eastern Europe, and Northwestern Russia) defined by the clustering of grid cells that experience hot spells at the same time. Temperatures are spatially averaged within each region to identify hot spells; the analysis specifically concentrates on events occurring in SW and W Europe. In SW Europe, persistent hot spells are tightly linked to antecedent soil dryness. Significant soil moisture anomalies are present in the weeks prior to and during the persistent hot spells but not prior to short hot spells. Persistent hot spells are associated with larger and higher-magnitude positive blocking frequency anomalies compared to short spells, as well as a significant positive frequency anomaly of cutoff lows upstream and south-west of the region, while the jet stream is shifted northwards. Large-scale anticyclonic Rossby wave breaking over Europe and the Mediterranean is also often associated with persistent hot spells in SW Europe. During short hot spells the upstream jet is located further south and the upstream wave train is more zonally oriented, indicating a more transient nature of the Rossby wave pattern, potentially leading to more variable surface weather. In W Europe, persistent hot spells are marked by strong land–atmosphere coupling, leading to intense soil desiccation during the events but no significant soil moisture anomalies prior to the events. A lower-wavenumber Rossby wave train compared to the short hot spells indicates a more stationary upper-level flow during persistent spells. High blocking frequency and recurrent Rossby wave packets feature in 87 % and 60 % of persistent events in this region, respectively. During short hot spells the upstream jet over the Atlantic extends further east and the upstream cyclone frequency is significantly higher than in the climatology, pointing to the important role of cyclones for the termination of short hot spells. In both regions, several dynamical mechanisms (blocking, recurrent Rossby wave packets, cutoff lows) contribute to persistent hot spells; in 80 % or more of the cases more than one type of mechanism was involved. The sequence of drivers during the persistent spells varies across spells. In both regions, short spells are associated with a configuration of the upper-level circulation that suggests the presence of more transient waves upstream over the North Atlantic.