Extracting the most persistent component of upward wave activity flux

Variation in the relative zonal phasing between a wave anomaly and the background stationary wave pattern—the "linear interference" effect—is known to explain a large part of planetary wave driving of the polar stratosphere in both hemispheres. Previous work has established that the component of upward wave activity flux that is linearly coherent with the stationary wave is more persistent than the total upward wave activity flux itself [Smith and Kusher, 2012]. We seek to understand the drivers behind this effect. Evidence in reanalysis data points to low frequency standing wave anomalies that are zonally aligned such that they primarily drive fluctuations in the amplitude of the stationary wave pattern. Although there are significant, vertically coherent, planetary-scale travelling waves in the troposphere and lower stratosphere, their structure and primary timescales (periods of around 25 days) mean that they play a relatively minor role in the variability of time-integrated upward wave activity flux. In addition, we find standing waves to be primarily responsible for the well-known connection between lower stratospheric vertical EP fluxes and fluctuations in the strength of the mid-stratosphere polar vortex strength. We discuss the possibility of longer lead-time prediction of extreme polar vortex strength events based on knowledge of the standing wave field, and therefore the seasonal prediction of tropospheric Northern Annular Mode related variables.