Predictability of European winter 2020/2021: Influence of a mid‐winter sudden stratospheric warming

Boreal winter (December–February) 2020/2021 in the North Atlantic/European region was characterised by a negative Atlantic Oscillation (NAO) index. Although this captured within ensemble spread of predictions from Met Office Global Seasonal forecast system (GloSea5), with 17% members predicting an NAO less than zero, mean shifted towards positive phase. The observed monthly anomalies were particularly January and February, following early sudden stratospheric warming (SSW), prolonged period Phase 6 or 7 Madden Julian (MJO) late January/early February. In contrast, showed expected teleconnection La Niña, signal resulting weakening Aleutian Low leading to reduction tropospheric wave activity, increase polar vortex strength reduced chance SSW. Forecasts initialised later season successfully predicted February once SSW MJO medium range timescale. GloSea5 likely over-predicted Niña which we estimate caused small bias probability. However, error is smaller uncertainty probability finite size, emphasising need for large ensembles. This case study also demonstrates advantage continuously updated lagged forecasts over ‘burst’ started on fixed date, since change due events can be detected promptly communicated users.