Dynamical Pathways to Persistent Summer Heat in Western Europe

Persistent high summer temperatures in Europe place substantial stress on natural and societal systems. Due to their prolonged duration, such hot spells can prevent these systems from finding respite, negatively impacting their ability to recover. The dynamics that cause persistent surface weather in the midlatitudes are the subject of active scientific debate, but more work is needed to understand the possible pathways leading to its persistence on subseasonal timescales. 

Our recent study compared long-lasting to short hot spells in Western Europe using data from the ERA5 reanalysis. We argue that, in addition to soil-temperature coupling, specific structural differences in atmospheric dynamics account for either more or less persistent weather. During long spells, the upstream Atlantic storm track may be weakened, stalled, or deflected north of the hot spell region. Small sample size and variability among the long spells, however, precludes making definitive statements on their typicality. Despite the commonalities in preconditions and large-scale patterns, the few observed historical cases exhibit a somewhat distinct sequence or combination of drivers, highlighting the potential diversity of dynamical pathways that can lead to persistent warm surface conditions. 

In the current analysis, we leverage data from the Community Earth System Model v2 Large Ensemble (CESM2-LE), increasing the number of events by two orders of magnitude compared to the previous study using the reanalysis. For two regions in Western Europe, we cluster geopotential height anomaly fields corresponding to persistent hot spells. This results in two archetypes of flow structures that sustain event duration: a) one dominated by large-scale Rossby wave breaking, a poleward shifted storm track and upstream cutoff lows, and b) another characterised by a more zonal wave train, stalled upstream cyclonic activity, and transient recurrent Rossby Wave packets (RRWPs). Additionally, we investigate where the ERA5 cases fit into the CESM2-LE archetypes and assess to what extent the ECMWF S2S ensemble re-forecasts are able to capture the duration and magnitude of the reanalysis events. 

By better understanding the mechanisms driving persistent surface weather extremes, we can inform how we approach their predictability, which is in turn crucial for informed decision-making and effective risk preparedness.