The predictability of the downward vs. non-downward propagation of sudden stratospheric warmings in S2S hindcasts

Roughly one-third of sudden stratospheric warming (SSW) events lack a strong canonical surface impact, and this can lead to a forecast bust if a strong impact had been predicted. Hence, it is important to predict before the SSW onset if an event will propagate downward. The predictability of the downward impact of SSWs is considered in 7 subseasonal-to-seasonal forecast models for 16 major SSWs between 1998 and 2022, a larger sample size than considered by previous works. The models successfully predict which SSWs have a stronger downward impact to 100hPa, however they struggle to predict which have a stronger tropospheric impact. The downward impact is stronger if the deceleration of the 10hPa winds is better predicted. Downward impact is stronger for split and for absorbing SSWs, and is better predicted in high-top models. In contrast, there is little relationship between SSWs with above-average predictability and the subsequent downward impact.