Tropopause-level planetary wave source and its role in two-waytroposphere–stratosphere coupling

Abstract. Atmospheric planetary waves play a fundamental role in driving stratospheric dynamics, including sudden stratospheric warming (SSW) events. It is well established that the bulk of the planetary wave activity originates near the surface. However, recent studies have pointed to a planetary wave source near the tropopause that may play an important role in the development of SSWs. Here we analyse the dynamical origin of this wave source and its impact on stratosphere–troposphere coupling, using an idealised model and a quasi-reanalysis. It is shown that the tropopause-level planetary wave source is associated with nonlinear wave–wave interactions and subsequent resonance as well as with transient wave decay. The resulting planetary waves may then propagate deep into the stratosphere, where they dissipate and may help to force SSWs. We find that when an SSW is preceded by the tropopause wave source, it is followed by a robust downward impact, decelerating the tropospheric zonal mean zonal winds between 40–60° N several weeks later. Unlike this robust response following the tropopause wave source events, we do not find such a robust downward impact following the SSWs preceded by surface wave source events. This suggests that the tropopause wave source could potentially be used as one of the predictors of not only SSWs, but their downward impact as well.