Vertical fluxes of wave activity from the troposphere to the stratosphere correlate strongly and negatively with the Northern Annular Mode index in the stratosphere and subsequently in the troposphere. Recent studies have shown that stratospheric NAM variability is negatively correlated with the amplitude of the wave pattern coherent with the climatological stationary wave field, particularly its wave-1 and wave-2 components; when the climatological stationary wave field is amplified or attenuated, the stratospheric jet correspondingly weakens or strengthens. Here we quantify the importance of this linear interference effect by performing a decomposition of the upward wave activity flux using reanalysis data. The interannual variability in upward 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 upward wave activity events reveals the significant role of linear interference, particularly for strong vortex events in the Northern Hemisphere. The persistence of the linear interference component of the vertical wave activity flux, corresponding to persistent phase or anti-phase locking between the climatological stationary wave and the wave anomaly, may help improve wintertime extratropical predictability.