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 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. Sudden stratospheric warming (SSW) refers to the rapid increase in temperature in the polar stratosphere during winter. It plays a crucial role in predicting weather patterns within a timeframe ranging from 2 weeks to 2 months (subseasonal-to-seasonal; S2S). This study investigates the factors that contribute to the predictability of the 2021 SSW. By comparing the S2S forecasts with and without the SSW, it is found that the predictability of SSW is not determined by the tropospheric precursors, but by the zonal-mean flow in the upper troposphere and lower stratosphere that modulates the wave propagation from the troposphere to the stratosphere. This result suggests that a better prediction of the wave propagation condition is a critical factor for a successful SSW prediction on the S2S time scale. The crucial factors that determine the predictability of the 2021 SSW in S2S prediction models are examinedThe k = 1 wave propagating into the stratosphere plays an essential role in the SSW predictionThe PV gradient in the upper troposphere and lower stratosphere can control the SSW predictability by modulating the upward wave propagation