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

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 days. 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 different techniques are needed for year-round applications.

Using ERA-Interim reanalysis, Grams (2017) introduced a year-round weather regime definition for the North-Atlantic European region which accounts for inter-seasonal differences by construction. Following established regime approaches they identified seven regime patterns based on 500 hPa geopotential height anomalies. The definition has been used in numerous studies for e.g. explaining surface weather modulation on multi-day time scales and the occurrence of extremes with applications, particularly in the energy sector. Furthermore the definition identifies objective regime life cycles facilitating process studies. These studies revealed dynamical behaviour of regime life cycles, and specific physical processes that determine regime life cycles, in particular those characterised by blocking. Finally, the predictability and forecast skill for the year-round regimes have been explored, linked to the representation of physical and dynamical processes, and pre-operational forecasting tools have been implemented.

This study now provides an update on ERA5 reanalysis data 1979-2019 and a thorough documentation of the seamingless year-round definition of seven North-Atlantic European weather regimes, accompanied with the open release of data and auxiliary scripts at Zenodo (Grams, 2025) for an easy start working with the regimes. First, the paper explains in detail the technical implementation of the weather regimes and why there is an optimal number of seven year-round regimes in the North Atlantic European region. Next, it shows similarities and differences to the canonical definition of seasonal weather regimes in Europe. A discussion of key characteristics follows, focussing on the inter-annual and intra-annual variability in regime occurrence, the duration of life cycles, regime transitions, and the modulation of surface weather and extremes. 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. The only exemption is Scandinavian Blocking which 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.