Understanding the Differences in the Sub-seasonal Predictability of Extreme Stratospheric Events: The extreme wave activity flux events in 2009 and 2018

Extreme stratospheric events such as sudden stratospheric warmings (SSWs) and strong vortex events can have downward impacts on surface weather that can last for several weeks to months. In subseasonal-to-seasonal (S2S) prediction systems, the predictability of these events strongly differs within events of the same type, and also between event types. The reason for the differences in predictability between events is however not resolved. To investigate this question using a larger sample size, we extend the definition of strong vortex and SSW events to wind acceleration and deceleration events due to their similar dynamics, respectively. Specifically, we use the zonal mean zonal wind at 60°N, 10hPa from ERA-interim reanalysis for the winters of 1998/99 to 2017/18 to identify wind acceleration and deceleration events, which are defined as a wind change over a 10-day window. We then assess the predictability of the identified events using the hindcasts from the ECMWF S2S prediction system. Overall,  wind acceleration events are found to be more predictable than deceleration events. However, when expressing the predictability of deceleration and acceleration events as a function of event magnitude, they qualitatively exhibit the same predictability behaviour; that is, events of stronger magnitude are less predictable. We explain the observed predictability dependence from two perspectives: 1) In a statistical sense, strong magnitude events lie within the tails of the climatological distribution and thus are penalised more heavily than weak magnitude events, and 2) from a dynamical perspective, extreme stratospheric events are associated with strong anomalies in precursors such as wave activity and vortex background state, which are themselves often associated with large ensemble spread and large uncertainties. The model shows a poor predictability of extreme wave activity fluxes in the lower stratosphere. In particular, the split SSW events in 2009 and 2018, which are reported to be associated with anomalously strong wave-2 activity flux, are found to exhibit large prediction errors even at short lead times, suggesting that nonlinear wave dynamics might play an important role.  We suggest that a better representation of extremely strong wave activity in the prediction system may enhance the predictability of stratospheric extreme events, and by extension their impacts on surface weather and climate.