The Predictability of the 2021 SSW Event Controlled by the Zonal‐Mean State in the Upper Troposphere and Lower Stratosphere

Abstract Sudden stratospheric warming (SSW) describes a disruption of the stratospheric polar vortex in the winter hemisphere. It affects not only the stratospheric circulation but also the surface climate for up to 2 months, serving as an important source of subseasonal‐to‐seasonal (S2S) predictability in midlatitudes. This study evaluates the predictability of the 2021 SSW and investigates the crucial factors that determine its predictability in the ECMWF and JMA S2S real‐time forecasts. In both models, only a subset of the ensemble members predicted the SSW at the lead time of about 2 weeks before the onset. By comparing the 10 ensembles with successful SSW predictions and those with failed predictions, we found that the ensembles predicting the SSW have relatively stronger wave fluxes from the upper troposphere to the stratosphere than the others. Stronger wave fluxes, particularly t…