Large-Scale Drivers of Persistent Extreme Weather During Early Summer 2021 in Europe

The early summer of 2021 was a season of extremes across Europe. Heatwaves, droughts and wildfires hit Eastern Europe and the Baltic, while repeated extreme precipitation in Western Europe culminated in massive floods in mid-July. The large-scale circulation during this period was remarkably persistent, with an extremely meridionally amplified flow over Europe. Recurrent blocking over the Baltic and Rossby wave breaking in the North Atlantic led to frequent heavy precipitation in Western Europe and the Black Sea and to warm and dry conditions over Eastern Europe. These conditions persisted for a month as the blocks and wave breaking episodes strengthened one another, while three closely spaced extratropical transitions of tropical cyclones in the eastern North Atlantic led to recurrent amplification of the jet. Seasonal anomalies thus emerge from the complex interactions of individual weather events, offering an interesting storyline for climate impact assessment and a formidable challenge for (sub-) seasonal prediction.