The work, carried out by Christian M. Grams of the Federal Office of Meteorology and Climatology MeteoSwiss, updates and thoroughly documents a regime definition first introduced in 2017. Where traditional European regime classifications were built separately for individual seasons, most famously the four canonical winter regimes identified in the 1990s, the year-round framework handles the entire calendar in one consistent system. The key trick is a normalization of anomalies in 500-hectopascal geopotential height, a standard measure of the large-scale flow at mid-altitudes of the atmosphere. Because winter anomalies are naturally much larger in amplitude than summer ones, an unnormalized analysis across all seasons would be dominated by winter. By dividing the low-pass filtered height anomalies by a seasonally varying measure of their climatological variability, the method removes the seasonal difference in amplitude while preserving the spatial shape of the flow, allowing summer and winter patterns to be compared on equal footing.

From this normalized data, an empirical orthogonal function analysis combined with fuzzy clustering identified seven distinct regimes, and the number seven was not chosen arbitrarily. The author tested configurations ranging from two to eleven clusters and used an objective similarity index, the maximum anomaly correlation coefficient between cluster patterns, to judge when clusters stopped being genuinely distinct. With four clusters the familiar canonical regimes reappear, but pushing to seven reveals finer structure while keeping the patterns well separated. Beyond seven, an eighth weak pattern emerges that closely resembles existing regimes, and the similarity between clusters jumps sharply. The seven regimes split into two families: three cyclonic regimes, Atlantic Trough, Zonal, and Scandinavian Trough, dominated by negative height anomalies and enhanced storm activity, and four blocked regimes, Atlantic Ridge, European Blocking, Scandinavian Blocking, and Greenland Blocking, dominated by positive anomalies and high pressure.

One of the study’s central contributions is an objective life cycle definition. Rather than simply labeling each day with a regime category, the method computes a continuous index for each regime, a standardized projection of the current atmospheric state onto the regime pattern. When this index exceeds a threshold of 1.0 for at least five days, a life cycle is identified, complete with onset, maximum, and decay stages. Crucially, life cycles can overlap, so one regime can be building while another is still dissolving, which matches how the atmosphere actually behaves. Roughly thirty percent of all time steps belong to no regime at all, corresponding to flow situations close to climatology. This continuous, dynamical view transforms regimes from a static classification into something closer to living entities with births, mature phases, and deaths.

The practical payoff of this framework is considerable. Weather regimes modulate surface conditions across entire continents for days to weeks, exactly the window where conventional weather forecasts lose skill but before seasonal averages take over. They shape wind speeds critical to Europe’s renewable power supply, including the dreaded Dunkelflaute episodes when low winds and weak sunshine coincide, and they modulate heat waves, cold spells, storm series, and atmospheric river landfalls. Previous work has linked prolonged heat-induced mortality to European blocking and winter stress on the UK health system to Greenland blocking. The new study shows that the most extreme regime life cycles, those lasting more than a month, coincide with Europe’s most extreme seasons. A Zonal regime life cycle in early 1990 lasted an extraordinary 73 days amid severe winter storms, while the longest Scandinavian Blocking life cycle began in July 2003 and lasted 32 days during Europe’s record-breaking heat. A 54-day Greenland Blocking episode starting in November 2010 brought a brutal cold wave.

The regimes also reveal a striking seasonal rhythm. Cyclonic regimes together account for up to forty percent of winter days but only about twenty-two percent in July, while blocked regimes dominate summer, exceeding fifty percent of days in June through August. European Blocking and Scandinavian Blocking alternate in preference: the former peaks in relative frequency in winter and spring, the latter in summer and autumn. Inter-annual variability is enormous. The early 1990s saw a run of strongly zonal years, 2010 brought a dominant negative North Atlantic Oscillation winter, and the hot years 2021 and 2022 featured frequent blocking. No single year matches the climatological average, underscoring how much year-to-year chaos overlays the seasonal cycle.

The most provocative question the study tackles is whether climate change is shifting regime frequencies. Extending the identification back to 1950 using the backward extension of the ERA5 reanalysis, the author finds that inter-annual variability dominates and significant trends are scarce, with one exception: Scandinavian Blocking shows a significant positive trend, concentrated in summer and autumn. But here the story takes a subtle turn. The 500-hectopascal geopotential height across the regime domain has been rising at about 5.4 geopotential meters per decade since 1979, consistent with the thermal expansion of the troposphere as the planet warms. When the regime identification is repeated with this area-averaged trend removed, the significant Scandinavian Blocking trend vanishes entirely. The apparent increase in summer blocking over Scandinavia, the analysis suggests, is likely primarily a thermodynamic signature of global warming rather than a genuine change in atmospheric circulation.

This finding raises what the author describes as an almost philosophical question: in a warmer climate, should a blocked regime be defined as a circulation anomaly deviating from the climatological background, or as a region of very high pressure with strong impacts on the surface? From an impact perspective, the thermodynamic effect of warming alone can already partly explain the recent increase in high-pressure episodes reported in some studies, and higher background temperatures likely amplify the local consequences of blocking, particularly for summer heat waves. The study also cautions that simple linear trend analyses are highly sensitive to the period chosen, and proposes that non-stationary methods, such as the LOESS filters increasingly used by European meteorological services, will be needed to cleanly separate natural variability from climate change signals in future regime research.

The paper is as much an infrastructure project as a scientific analysis. It provides a comprehensive technical appendix documenting every step of the method, from the Lanczos low-pass filtering to the iterative life cycle identification criteria, and the complete regime dataset from 1950 to the present has been openly released on Zenodo along with auxiliary scripts. The author shows that switching from the older ERA-Interim reanalysis to ERA5 changes the attribution of only a tiny fraction of time steps, confirming the robustness of the framework. With ECMWF and MeteoSwiss now working toward operational use of the year-round regimes in sub-seasonal forecasting and climate monitoring, these seven atmospheric archetypes are poised to move from the research literature into the daily toolkit of forecasters, energy traders, and health planners across Europe.

Subject of Research: Year-round weather regimes and their life cycles, variability, and trends in the North Atlantic European region

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

Article References: Grams, C. M. (2026). A life cycle definition of year-round weather regimes in the North Atlantic European region. Weather and Climate Dynamics, 7(3), 1641-1680. https://doi.org/10.5194/wcd-7-1641-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1641-2026

Keywords: weather regimes, atmospheric blocking, North Atlantic, Europe, ERA5 reanalysis, sub-seasonal forecasting, weather regime life cycles, Scandinavian Blocking, climate trends, geopotential height, heat waves, renewable energy