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

Abstract

AbstractSudden 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 those of zonal wavenumber one, are not the result of the tropospheric precursors such as the Ural blocking and Aleutian cyclones but they result from the modulation of the wave propagation by the background state. In particular, the ensembles with failed SSW predictions tend to have a negative potential vorticity gradient in the upper troposphere and lower stratosphere, which limits the upward wave propagation into the stratosphere and provides an unfavorable condition for the SSW. This result suggests that not only the wave sources in the troposphere but also the background state in the upper troposphere and lower stratosphere can modulate the predictability of SSW in S2S prediction models.