Verification of Subseasonal-to-Seasonal Forecasts for Major Stratospheric Sudden Warmings in Northern Winter from 1998/99 to 2012/13

This study reports verification results of hindcast data of four systems in the subseasonal-to-seasonal(S2S)prediction project for major stratospheric sudden warmings(MSSWs)in northern winter from 1998/99 to 2012/13.This report deals with average features across all MSSWs,and possible differences between two MSSW types(vortex displacement and split types).Results for the average features show that stratospheric forecast verifications,when further averaged among the four systems,are judged to be successful for lead times around 10 d or shorter.All systems are skillful for lead times around 5 d,whereas the results vary among the systems for longer lead times.A comparison between the MSSW types overall suggests larger forecast errors or lower skill for MSSWs of the vortex split type,although the differences do not have strong statistical significance for almost all cases.This limitation is likely to at least partly reflect the small sample size of the MSSWs available.

Seasonal timing of stratospheric final warming associated with the intensity of stratospheric sudden warming in preceding winter

The association of seasonal timing of stratospheric final warming events(SFWs) in spring and the occurrence of major and minor stratospheric sudden warming events(SSWs) in midwinter were investigated through statistical analysis, parallel comparison, and composite analysis, based on the NCEP-NCAR reanalysis dataset covering 1958–2012. It was found that the intensity and occurrence of winter SSW events can largely affect the timing of spring SFWs. Specifically, the SFW onset dates tend to be later(earlier) after the occurrence(absence) of winter major SSWs. However, the occurrence or absence of minor SSWs does not change the frequency of early and late SFWs. A parallel comparison of the temporal evolution of the anomalous circulation and planetary-waves between major SSW and minor SSW winters indicates that the stratospheric polar vortex(polar jet) will keep being anomalously stronger 30 days after major SSW onset. And the associated significant negative Eliassen-Palm(EP) flux anomalies can persist for as long as 45 days after major SSW events. In contrast, the circulation anomalies around the occurrence of minor SSW events can last only a few days. To further verify the possible influence of the occurrence of major SSWs on the seasonal timing of SFWs, composite analysis was performed respectively for the 21 major-SSW years, 15 minor-SSW years, and the 15 non-SSW years. Generally, planetary-wave activity in the extratropical stratosphere tends to be stronger(weaker) and the westerly polar jet is anomalously weaker(stronger) in major-SSW(non-SSW) winters. But in the following spring, the planetary-wave activity is weaker(stronger) accompanied with an anomalously stronger(weaker) stratospheric polar vortex. In spring after minor-SSW years, however, the stratospheric polar vortex and the westerly polar jet exhibit a state close to climatology with relatively gentle variations.