Long vs. Short: Understanding the dynamics of persistent summer hot spells in Europe

Abstract. 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 days) and short-lived (4–5 days) regional-scale hot spells over Europe during summer using the ERA5 reanalysis. We identify six coherent regions in Europe (Southwestern Europe, Western Europe, Central-Southern Europe, Northern Europe, Eastern Europe and Northwestern Russia) defined by the clustering of gridcells which experience long-lasting hot spells at the same time. Temperatures are averaged across these regions for an analysis of hot spells 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 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 wavebreaking over Europe and the Mediterranean is also often associated with persistent hot spells in SW Europe. During short spells the upstream jet is located further south and the upstream wavetrain is more zonally oriented, pointing to a more transient large-scale upstream flow configuration matching the more transient nature of the spells.

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 wavetrain compared to the short spells indicates a more stationary upper-level flow during persistent spells. High blocking frequency and recurrent Rossby wave packets (RRWPs) feature in 87 % and 60 % of persistent events in this region, respectively. During short 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, RRWPs, cutoff lows) are contributing to persistent 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 more transient flow situation upstream over the North Atlantic.