The role of standing waves in driving persistent anomalies of upward wave activity flux

The timescale of vertical wave activity pulses is known to play a key role in their impact on the stratospheric circulation. It has been shown that pulses of length greater than 10 days are most likely to drive a sudden stratospheric warming (SSW) like response, with a downward propagation of zonal mean wind anomalies through the stratosphere [Harnik 2009; Sjoberg and Birner 2012, 2014]. We investigate the mechanisms underlying persistent wave activity flux pulses. Using the linear interference framework, we establish that the component of upward wave activity flux that is linearly coherent with the stationary wave pattern is more persistent than the total wave activity flux itself. Decomposing the wave anomaly into standing and travelling parts, we show that low frequency standing waves are primarily responsible for this persistence. These standing waves have preferred zonal positions, and tend to reinforce and attenuate the climatological wave. Furthermore, we find that standing waves drive the connection between vertical wave activity flux anomalies and polar vortex strength. This holds for both correlations over all days, and during extreme events such as SSWs. The low frequencies of the standing waves suggests a possibility for longer lead-time prediction of extreme stratospheric circulation changes, and hence subseasonal prediction of tropospheric Northern Annular Mode related variables.