The role of momentum forcing by planetary-scale waves in the onset of tropospheric circulation anomalies during stratospheric sudden warming events

Stratospheric Sudden Warming (SSW) events are well known for their dramatic weakening of the stratospheric polar vortex, often leading to a reversal of the zonal circulation, but also for the tropospheric circulation anomalies that often accompany the events. Although recent evidence points to the breaking of planetary-scale waves as the dominant mechanism for the deceleration of the stratospheric vortex, the mechanisms producing the tropospheric anomalies remain uncertain. Several dynamical mechanisms have been suggested to explain the apparent coupling between the stratosphere and troposphere during the events including downward control, tropospheric wave feedback processes, wave reflection or complex interactions among these. However, the relative roles played by those mechanisms remain uncertain. To improve our understanding of processes leading to the coupling, we focus our attention on the short term response of the troposphere corresponding to the onset of the tropospheric anomalies. Using ERA-Interim dataset and transformed Eulerian mean diagnostics of momentum forcing based on the primitive equation set, we evaluate the role of different forcing terms in explaining the tropospheric wind tendencies in a composite of SSW events. Our results point to the importance of momentum fluxes induced by planetary-scale waves as their activity intensifies in the troposphere prior to the vortex breakdown. Enhanced equatorward momentum fluxes in the polar region by wavenumber 1 and 2 play a major role in decelerating tropospheric zonal wind north of 60°N. Further separation of the events according to the timing of tropospheric deceleration reveals that different wave scales (wavenumber-1 dominated vs wavenumber-2 dominated events) affect the timing of the tropospheric anomalies, with wavenumber-2 events presenting tropospheric deceleration before the weakening of the vortex. These new results suggest that the direct effect of planetary-scale waves on the tropospheric flow not only play a major role for the onset of tropospheric anomalies during SSW events but is also a significant source of variability.