4:00 PM - 4:15 PM
[AAS10-09] A Statistical Analysis of Downward Planetary Wave Propagation in the Eastern Hemisphere Stratosphere During Boreal Winter Based on Large Ensemble Data
Keywords:stratosphere, planetary waves, wave activity flux
It is difficult to make a statistical analysis of downward planetary wave propagation in the Eastern Hemisphere stratosphere during boreal winter based on observational data, because the event number is low. So, the statistical features of those events remain unknown, but large ensemble data, d4PDF (Mizuta et al., 2017), allows us to make such analyses. Hence, we statistically investigate the events using d4PDF. Thus, the purpose of this study is to clarify the features, causation mechanisms, and effects on the tropospheric circulation of the downward propagation events in the Eastern Hemisphere stratosphere.
We extract the significant downward propagation events based on the vertical component of Eliassen-Palm flux (Andrews et al., 1987). We define the “Eastern Hemisphere events” as those events with the negative peak of the vertical component of Plumb (1985) 's wave activity flux over 45 °E through 90 °E and determine the “event days” that the negative peaks occur. Resultantly, we successfully obtain 432 events. It is found that the wave packets propagate upward in the Western Hemisphere four days before the event days from the lag-composite analysis based on the event days. Westerly winds are decelerated over the same longitudinal region as the downward propagation occurrence in the upper stratosphere just after the events. Moreover, statistically significant cyclonic anomalies are found to be developed in the same longitude region as the downward propagation attains in the troposphere. These results reveal that the occurrence of the downward propagation in the Eastern Hemisphere stratosphere requires the upward propagation in the Western Hemisphere bringing the westerly wind deceleration prior to the events and that such events affect the development of cyclonic anomalies in the troposphere in the same longitude region. These features are similar to those of the events occurring in the Western Hemisphere stratosphere, although the location and frequency are different.
We extract the significant downward propagation events based on the vertical component of Eliassen-Palm flux (Andrews et al., 1987). We define the “Eastern Hemisphere events” as those events with the negative peak of the vertical component of Plumb (1985) 's wave activity flux over 45 °E through 90 °E and determine the “event days” that the negative peaks occur. Resultantly, we successfully obtain 432 events. It is found that the wave packets propagate upward in the Western Hemisphere four days before the event days from the lag-composite analysis based on the event days. Westerly winds are decelerated over the same longitudinal region as the downward propagation occurrence in the upper stratosphere just after the events. Moreover, statistically significant cyclonic anomalies are found to be developed in the same longitude region as the downward propagation attains in the troposphere. These results reveal that the occurrence of the downward propagation in the Eastern Hemisphere stratosphere requires the upward propagation in the Western Hemisphere bringing the westerly wind deceleration prior to the events and that such events affect the development of cyclonic anomalies in the troposphere in the same longitude region. These features are similar to those of the events occurring in the Western Hemisphere stratosphere, although the location and frequency are different.