Year?round sub?seasonal forecast skill for Atlantic–European weather regimes

Weather regime forecasts are a prominent use case of sub-seasonal prediction in the midlatitudes. A systematic evaluation and understanding year-round forecast performance is still missing, however. Here we evaluate representation skill for seven Atlantic–European weather regimes reforecasts from European Centre Medium-Range Forecasts. Forecast calibration improves frequency biases most strongly summer, but scarcely winter, due to considerable large-scale flow summer. The average horizon winter about 5 days longer than summer spring, 3 autumn. Zonal Regime Greenland Blocking tend have longest horizon, which driven by their high persistence winter. lowest Blocking, common all seasons pronounced spring. For related, more northern Scandinavian similarly low spring higher We further show that tends be enhanced following strong stratospheric polar vortex (SPV), reduced weak SPV. Likewise, phases 4 7 Madden–Julian Oscillation (MJO) phase 2, also autumn Our study thus reveals promising potential predictions. Further model improvements can achieved reduction better modeling blocking region, exploitation predictability provided SPV states specific MJO transition seasons.