Stratospheric sudden warming as a threshold behavior of Rossby waves

We present evidence that stratospheric sudden warmings (SSWs) are, on average, a threshold behavior of finite-amplitude Rossby waves arising from wave-mean flow interaction. Competition between an increasing wave activity and a decreasing zonal-mean zonal wind sets a limit to the upward wave activity flux of a stationary Rossby wave.  A rapid, spontaneous vortex breakdown occurs once the upwelling wave activity flux reaches the limit, or equivalently, once the zonal-mean zonal wind drops below a certain fraction of the wave-free, reference-state wind obtained from the zonalized quasigeostrophic potential vorticity.  This threshold faction is 0.5 in theory and about 0.3 in reanalyses.  We use the ratio of the zonal-mean zonal wind to the reference-state wind as a local, instantaneous measure of the proximity to vortex breakdown, i.e. preconditioning.  The ratio generally stays above the threshold during strong-vortex winters until a pronounced final warming, whereas during weak-vortex winters it approaches the threshold early in the season, culminating in a precipitous drop in midwinter as SSWs form. The essence of the threshold behavior is captured by a semiempirical 1D model of SSWs, analogous to the “traffic jam” model of Nakamura and Huang for atmospheric blocking. This model predicts salient features of SSWs including rapid vortex breakdown and downward migration of the wave activity/zonal wind anomalies, with analytical expressions for the respective timescales. Model’s response to a variety of transient wave forcing and damping is discussed.