Preconditioned Stratospheric Modulation on the Occurrence of Stratospheric Final Warmings

Stratospheric Final Warmings (SFWs) are considered to have a distinct impact on the surface weather and stratospheric ozone. Most SFWs are predominantly wave-driven, while previous studies generally focused on the effects of tropospheric anomalous wave forcing on stratospheric wave amplification. This study demonstrates the crucial role of preconditioned stratospheric modulation on the occurrence of wave-driven SFWs in the Northern Hemisphere, using ERA5 reanalysis. In most cases, the preconditioned stratospheric state is the key factor in determining the occurrence of both early and late SFWs. The tropospheric anomalous wave forcing cannot independently trigger most SFWs without stratospheric modulation. The proportion of anomalous stratospheric signals followed by SFWs is much higher than that of anomalous tropospheric forcing, even if the proportions of SFWs preceded by anomalous stratospheric signals and tropospheric wave forcing are similar. Additionally, about 25% of SFWs are only preceded by stratospheric modulation without anomalously strong tropospheric wave amplification. Each Spring, the extratropical-stratospheric circumpolar westerly called the polar vortex, reverses to summertime easterly in the Northern Hemisphere. This seasonal transition is known as stratospheric final warmings (SFWs). SFWs have been suggested to exert a notable impact on the tropospheric circulation and stratospheric ozone. There is considerable interannual variability in the timing of SFWs, from as early as mid-March to as late as May. The timing of SFWs is crucial for subseasonal to seasonal climate predictability. Early SFWs and many late SFWs are predominantly driven by stratospheric wave forcing. Climatologically, the stratospheric planetary waves originate from the troposphere. Thus, the stratospheric wave amplification in SFWs is generally attributed to the effects of anomalous wave sources in the troposphere. However, our results suggest that the modulation of preconditioned stratospheric signals is the key factor in the occurrence of both early and late SFWs. The stratospheric modulation is not less important for triggering SFWs than the anomalous wave forcing from the troposphere. The modulation of preconditioned stratospheric state on stratospheric wave amplification is a key factor to trigger wave-driven stratospheric final warmings (SFWs)More than half of SFWs is preceded by anomalously strong stratospheric signalsAbout 25% of SFWs are only preceded by robust stratospheric signals and without anomalously strong tropospheric wave forcing