Characteristics of planetary wave activity of downward-propagating and non-downward-propagating stratospheric weak polar vortex events

The downward propagation of circulation anomaly signals associated with stratospheric weak polar vortex events can significantly affect the large-scale circulation in the mid-high latitudes of the troposphere, thereby modulating weather and climate variability. Understanding the influencing factors and key processes of the downward propagation characteristics of these anomalous signals is crucial for the application of stratospheric signals in extended-range forecasting in the troposphere. Based on the vertical evolution characteristics of the zonal-mean zonal wind anomalies averaged over 50°N-70°N, a total of 60 winter (November-March) stratospheric weak polar vortex events during 1952-2022 were classified into downward-propagating and non-downward-propagating events. Events that propagated down to 500 hPa (downward-propagating type) accounted for approximately 53.3% of the total, with 43.3% even propagating down to 1000 hPa. The differences in stratospheric polar vortex characteristics between the two types of events mainly manifested as follows: compared with non-downward-propagating events, downward-propagating events were accompanied by stronger peak intensities and longer durations of circumpolar westerly anomalies, while the differences in the morphology and position of the stratospheric polar vortex were not significant. Diagnostic analyses of planetary wave activity showed that the upward and poleward Eliassen-Palm (E-P) flux anomalies in the early stage of downward-propagating weak polar vortex events were generally stronger than those in non-downward-propagating events. This was related to the weak westerly conditions in the mid-high latitudes of the upper troposphere in the early stage of downward-propagating events, which led to wave propagation environments more conducive to the upward propagation of planetary wave energy, and was the dynamic reason why the intensity and duration of stratospheric anomalies in downward-propagating events exceeded those in non-downward-propagating events. In the late stage of weak polar vortex events, the downward and poleward E-P flux anomalies associated with downward-propagating events were generally stronger than those in non-downward-propagating events. This was related to wave propagation environments from the lower stratosphere to the middle troposphere being more conducive to the reflection of planetary waves 1-2 (especially wave 1), as well as the weakening of the polar jet stream accompanied by the strengthening of the tropospheric subtropical jet stream, which restricted more tropospheric planetary waves to mid-high latitudes. Anomalous wave reflection from the stratosphere to the troposphere and the jet stream conditions in the troposphere that are conducive to planetary wave development explain the late-stage downward propagation characteristics of such events.