Atmospheric blocking refers to quasi-stationary, large-scale atmospheric patterns characterized by a dominant anticyclonic anomaly that effectively “blocks” or redirects midlatitude weather systems. Despite progress in numerical weather prediction, blocking remains difficult for weather and climate models to represent due to the complex multi-scale processes involved in its lifecycle. Moreover, the persistence of these patterns frequently triggers extreme surface weather, including prolonged heatwaves, cold spells, droughts, heavy precipitation events, and associated flooding. While recent studies highlight the importance of latent heat release in building and maintaining the upper-level anticyclonic anomaly, different perspectives attribute varying roles to dry and moist dynamics. It also remains unclear whether their relative contributions differ across regions where blocking occurs.

The first part of the seminar introduces the quasi-Lagrangian potential vorticity (PV) framework, which tracks negative upper-tropospheric PV anomalies associated with blocking and quantifies the processes that govern changes in block amplitude. This approach enables an assessment of the respective roles of dry and moist dynamics. The framework has been applied to the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis dataset (1979–2021). In the second part of the seminar, the quasi-Lagrangian framework is applied to blocks embedded within four anticyclonically dominated weather regime types in the North Atlantic–European region: Greenland Blocking, Atlantic Ridge, European Blocking, and Scandinavian Blocking. The focus is placed on the pre-history of blocks prior to blocked regime onset and on the role of dry and moist processes in the amplification of regime-related PV anomalies. Finally, the third part presents ongoing work examining how the underlying dynamics (dry versus moist) differ among four Northern Hemisphere blocking types: Omega blocking, dipole (Rex) blocking, anticyclonic Rossby wave breaking (RWB), and cyclonic RWB.

Seraphine Hauser, ETH Zürich.

Lieu
École normale supérieure – PSL
24 rue Lhomond – aile Erasme
salle Claude Froidevaux – E314