A life cycle definition of year-round weather regimes in the North Atlantic European region

Abstract

Abstract. Weather regimes are quasi-stationary, persistent, and recurrent states of the large-scale extratropical circulation. Weather regimes explain most of the multi-day atmospheric variability on sub-seasonal time scales of 5 to 30 d. While regime definitions have been explored for the European region extensively, in recent years the existence of regimes in other world regions such as North and South America, and East Asia has been confirmed. Importantly, traditional regime definitions focus on a specific season and adapted approaches are needed for year-round applications. Using ERA-Interim reanalysis, Grams et al. (2017) introduced a year-round weather regime definition for the North Atlantic European region which accounts for inter-seasonal differences by construction. The study at hand now provides an update on ERA5 reanalysis data 1979–2019. It newly discusses commonalities, differences, and the rationale behind year-round North Atlantic European regimes compared to the canonical seasonal regimes, and presents a general overview of regime characteristics. The emphasis lays on inter-annual and intra-annual variability of regime occurrence. It is shown that the most extreme weather regime life cycles in terms of duration go along with extreme seasons in Europe, featuring heat waves, cold spells, or storm series. Finally, potential trends in regime occurrence are explored by extending the regime identification to the period 1950–2024. Overall inter-annual variability of regime occurrence dominates and there are hardly significant trends. Only Scandinavian Blocking shows a significant positive trend in summer and autumn in line with expected trends. The trend can be related to the thermal expansion of the troposphere under global warming but is highly sensitive to the methodology used. Next to present new insight in regime occurrence and trends, the paper aims to serve as a basis for subsequent work. It therefore also represents a thorough documentation of the seamless year-round definition of seven North Atlantic European weather regimes, and, accompanied with the open release of data and auxiliary scripts at Zenodo (Grams, 2025), it facilitates an easy start working with the year-round regimes.