Year-round North Atlantic-European Weather Regimes in ERA5 reanalyses

This dataset contains the original files for a life-cycle definition of 7+1 year-round North Atlantic-European weather regimes following the definition of Grams et al. (2017, https://doi.org/10.1038/nclimate3338), updated to ECMWF ERA5 reanalysis as described in Hauser et al. (2024, https://doi.org/10.5194/wcd-5-633-2024). It is supplementary data to the publication Grams (2026, in review for Weather Clim. Dynam. https://doi.org/10.5194/egusphere-2025-6385) which contains an in-depth technical explanation and analysis of key characteristics and trends in these regimes. A key novelty in the regime definition are the objectively identified regime life cycles which allow process-oriented predictability studies (e.g. Hauser et al. 2024). 

The dataset has been originally computed for the data period 1979-2019. Using the backward extension of ERA5 (Soci et al., 2024, https://doi.org/10.1002/qj.4803), it has been extended to 1950. Using the near-realtime update it is extended forward until 20250726_21 in release V1.0. For the lifetime of ERA5 it is planned to continously update the data irregularly at least once per year and upon request. Future updates will ensure full backward compatibility.

For the methodological documentation we refer to Grams (2026). 

This dataset also contains a Juypter notebook with examples of simple data analysis in python. The notebook aims to facilitate an easy start of the work with the data. Christian thanks Seraphine Hauser and Dominik Büeler for help with coding this ipynb and reformatting the data for easier accessibility. Users are advised to get familiar with this `Readme.md` prior to using this data. 

Contact: weatherregimes@gmail.com (Christian Grams)

Bell, B., Hersbach, H., Simmons, A., Berrisford, P., Dahlgren, P., Horányi, A., et al.: The ERA5 global reanalysis: Preliminary extension to 1950. Q J R Meteorol Soc, 147(741), 4186–4227, https://doi.org/10.1002/qj.4174, 2021.

Grams, C. M.: A life cycle definition of year-round weather regimes in the North Atlantic European region, EGUsphere [preprint], in review for Weather Clim. Dynam.,  https://doi.org/10.5194/egusphere-2025-6385, 2026.

Grams, C. M., Beerli, R., Pfenninger, S., Staffell, I., and Wernli, H.: Balancing Europe's wind power output through spatial deployment informed by weather regimes, Nat. Clim. Change, 7, 557–562, https://doi.org/10.1038/nclimate3338, 2017.

Hauser, S., Teubler, F., Riemer, M., Knippertz, P., and Grams, C. M.: Life cycle dynamics of Greenland blocking from a potential vorticity perspective, Weather Clim. Dynam., 5, 633–658, https://doi.org/10.5194/wcd-5-633-2024, 2024. 

Hersbach H, Bell B, Berrisford P, et al.: The ERA5 global reanalysis. Q J R Meteorol Soc, 146: 1999–2049. https://doi.org/10.1002/qj.3803, 2020. 

Soci, C., Hersbach, H., Simmons, A., Poli, P., Bell, B., Berrisford, P., et al.: The ERA5 global reanalysis from 1940 to 2022. Q J R Meteorol Soc, 150(764), 4014–4048, https://doi.org/10.1002/qj.4803, 2024.