Linear interference and the initiation of extratropical stratosphere-troposphere interactions

Vertical fluxes of wave activity from the troposphere to the stratosphere correlate negatively with the Northern Annular Mode (NAM) index in the stratosphere and subsequently in the troposphere. Recent studies have shown that stratospheric NAM variability is also negatively correlated with the amplitude of the wave pattern coherent with the large-scale climatological stationary wavefield; when the climatological stationary wavefield is amplified or attenuated, the stratospheric jet correspondingly weakens or strengthens. Here we quantify the importance of this linear interference effect in initiating stratosphere-troposphere interactions by performing a decomposition of the vertical wave activity flux using reanalysis data. The interannual variability in vertical wave activity flux in both the Northern and Southern Hemisphere extratropics is dominated by linear interference of quasi-stationary waves during the season of strongest stratosphere-troposphere coupling. Composite analysis of anomalous vertical wave activity flux events reveals the significant role of linear interference and shows that "linear" and "nonlinear" events are essentially independent. Linear interference is the dominant contribution to the vertical wave activity flux anomalies preceding displacement stratospheric sudden warmings (SSWs) while split SSWs are preceded by nonlinear wave activity flux anomalies. Wave activity variability controls the timing of stratospheric final warmings, and this variability is shown to be dominated by linear interference, particularly in the Southern Hemisphere. The persistence of the linear interference component of the vertical wave activity flux, corresponding to persistent constructive or destructive interference between the wave-1 component of climatological stationary wave and the wave anomaly, may help improve wintertime extratropical predictability.