Ensemble mean chiclet plot (last 90 runs)
NAO+ = Positive North Atlantic Oscillation, AR = Atlantic Ridge, NAO– = Negative North Atlantic Oscillation, SB = Scandinavian Blocking, NO = No Regime
Bars show the proportion of ensemble members assigned to each regime. Squares across the top show the attribution of the ensemble mean, which often tends toward No Regime at long lead-times. The weather regime index (WRI) is the standardised amplitude of each pattern.
About the forecasts
These forecasts use the 46-day 101-member extended-range run of the ECMWF Integrated Forecasting System (IFS). The regimes are computed using 00Z 500 hPa geopotential height (Z500) anomalies in the domain 20–80°N, 90°W–30°E (following Cassou 2008). The forecast data are obtained from the S2S archive via the ECWMF Public Datasets service with a 1.0° grid. The latest available forecast is from 2 days beforehand. The method partly follows that used to define winter regimes in Cassou (2008) but is extended to year-round following the method of Lee et al. (2023) and Lee and Messori (2024). The regime centroids are computed over 1979–2025 using ERA5. The forecast Z500 data are smoothed with a 5-day centred running-mean prior to regime assignment. This is not the same regime definition used in ECMWF’s product so the regime probabilities will differ. Note that no mean-state hindcast bias correction of the Z500 field is performed, which may affect the regime assignment at longer lead-times.
If the anomalies are closer to climatology than a regime centroid by Euclidean distance, then “No Regime” is assigned. A map of the average Z500 anomalies associated with the regimes is available here.
If you have any queries, issues, or suggestions, please let me know. The script which produces these plots is currently scheduled to run at 0700 UK time.