Onset of circulation anomalies during stratospheric vortex weakening events: the role of planetary-scale waves

While mounting evidence links the Stratospheric Vortex Weakening (SVW) events to the upward propagation of planetary-scale Rossby waves from the troposphere to the stratosphere, the causes of the accompanying tropospheric circulation anomalies remain uncertain. To highlight the details of stratosphere-troposphere dynamical coupling during the onset of SVW events, this study identifies SVW events using temporal vortex evolution and performs composite budget analyses of the zonal wind tendency in the Transformed Eulerian Mean (TEM) framework on daily time scales. The time evolution of zonal wind anomalies shows a near-instantaneous vertical coupling in a time scale of only a few days. The zonal wind changes in the stratosphere and troposphere are caused by an anomalous upward and poleward propagation of zonal wavenumber one and two waves, with tropospheric changes dominated by the latter. Diagnostics of finite amplitude wave activity suggest that SVW events are caused mainly by the growth of planetary-scale waves rather than by wave breaking. It is also found that wavenumber-one disturbances in the troposphere have less geographical preference during the onset of the SVW events, whereas wavenumber-two disturbances project strongly onto the climatological pattern of planetary-scale waves in most cases. This results in a constructive interference that systematically modulates vertically propagating planetary-scale waves into the stratosphere. Impact of the background flow and sensitivity to the definition of SVW are also discussed.