Flow-dependent Sub-seasonal Forecast Skill for Atlantic-European Weather Regimes

Abstract:

The continuous increase of computational power and improvement of numerical weather prediction systems in recent decades has allowed extending the operational weather forecast horizon into sub-seasonal time scales (10 – 60 days). On these scales, quasi-stationary, persistent, and recurrent large-scale flow patterns, so-called weather regimes, explain most of the regional surface weather variability in the midlatitudes and are thus of primary interest in sub-seasonal forecasting for the respective region. Here, we assess the skill of sub-seasonal reforecasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) for predicting 7 year-round weather regimes in the Atlantic-European region. We primarily show that forecast skill considerably differs for different flow situations and seasons. We further elucidate how forecast skill is affected by classical compared to flow-dependent model calibration techniques. Finally, we give insight into how lower-frequency climate modes such as the stratospheric polar vortex as well as midlatitude synoptic-scale activity such as warm conveyor belts may enhance or dilute flow-dependent forecast skill.