Blocking patterns are crucial in producing recent extreme summer floods

Extreme weather events often, but not exclusively, occur when the jet stream is highly disturbed and the atmospheric circulation becomes blocked, allowing long-lasting, quasi-stationary and self-sustaining atmospheric weather regimes to develop. The interactions of subtropical, warm and moist air with polar, cold and dry air within the structure of the atmospheric block may then provide the local ingredients for these highly impactful weather events, including persistent rainfall from cut-off low pressure systems causing floods like those in Central Europe in 2024, or in Germany in 2021, or in Greece or Spain in 2023, or short-duration downbursts leading to serious flash flooding as occurred in Liguria, Italy in Oct 2023 breaking the European record for hourly rainfall. This talk will draw on evidence from several published and unpublished studies to examine the mechanisms for such events, from global drivers, through synoptic scale weather regimes to local-scale processes. Identifying the causal pathways for hydroclimatic extremes is important for developing improved methods for event attribution, and for improving climate model projections, since even high-resolution climate models poorly simulate key mechanisms driving these events and likely underestimate future changes.