Predictability of stratospheric sudden warmings based on a simple threshold metric

Recently we have demonstrated that stratospheric sudden warmings (SSWs) may be understood as a threshold behavior of finite-amplitude Rossby waves arising from wave-mean flow interaction [1]. The competition between an increasing wave activity and a decreasing zonal-mean zonal wind imposes an upper bound on the wave activity flux density that a stationary Rossby wave can transmit upward, and this upper bound decreases exponentially with an increasing altitude. Thus the capacity of the circumpolar jet as a conduit of upward Rossby wave transmission decreases rapidly with height. When the wave activity flux reaches the capacity at certain altitude, the excess flux is channeled into enhancing wave activity and decelerating the zonal-mean zonal wind, leading to a rapid vortex breakdown. It has been shown that at any altitude, the maximum wave activity flux is reached when the instantaneous zonal-mean zonal-wind $\overline u$ decreases to a fraction of a hypothetical eddy-free reference state wind $u_{REF}$. On average, this critical ratio of $\overline u/u_{REF}$ (= rc) is about 0.3 for reanalysis data. The present study follows up on our previous work and examines the utility of rc as a predictor of SSWs. By examining the winter time series of rc at z = 32 km (~10 hPa) at 60ºN for ERA5 [2] and the 100-year output of a GCM, we find that more than half of the time the zonal-mean zonal wind at this location reverses to easterly in a few days after rc drops below the threshold value, but there are a significant number of cases in which the wind reversal does not occur even rc crosses the threshold. To be sure, even for these false positive cases, the zonal-mean zonal wind is decelerated significantly (minor SSWs), but the vortex recovers before the wind direction reverses. It is demonstrated that the false positives are generally characterized by a short-lived upward wave activity flux, but the value of rc itself is not a good predictor of the duration of the wave activity flux. Overall, rc has a statistically signifiant predictive skill for significant vortex-weakening events, but it should not be considered a sufficient condition for an SSW. References [1] Nakamura et al. 2020, JAS: https://doi.org/10.1175/JAS-D-19-0249.1 [2] Hersbach, H. et al., 2020. https://doi.org/10.1002/qj.3803