The Predictability of the Downward Versus Non‐Downward Propagation of Sudden Stratospheric Warmings in S2S Hindcasts.

Roughly one‐third of sudden stratospheric warming (SSW) events lack a strong canonical surface response, and this can lead to a forecast bust if a strong response was predicted. Hence, it is desirable to predict before SSW onset if an event will propagate downward. The predictability of the downward response of SSWs is considered in seven 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 before SSW onset which SSWs have a stronger downward response to 100 hPa, however they struggle to predict which have a stronger tropospheric response. The downward response is stronger if the magnitude of the deceleration of the 10 hPa winds is more accurately predicted. Downward response is stronger for split and absorbing SSWs. In contrast, there is little relationship between SSWs whose onset can be predicted at earlier leads and the downward response. Plain Language Summary: The wintertime stratosphere typically features circumpolar strong westerly winds, but on occasion these strong winds can reverse and temperatures over the pole can rise by tens of degrees in an event known as a sudden stratospheric warming (SSW). Such an event increases the likelihood of extreme cold over Northern Eurasia and wet conditions in Southern Europe, however roughly a third of events do not feature such downward propagation. Sixteen SSW events have occurred in the Northern Hemisphere over the period 1998 to 2022, and this study considers whether the models that have contributed to the subseasonal to seasonal (S2S) database are able to distinguish which are downward propagating and which are not. We also explore the factors that govern downward propagation of the SSW signal in these models. Key Points: Downward versus non‐downward response of SSWs to 100 hPa is predicted by subseasonal prediction models ∼20 days before SSW onsetIn contrast, models struggle to predict the presence/absence of a tropospheric response before SSW onsetDownward response and predictability of SSWs unrelated; split and absorbing SSWs show stronger downward response in models and reanalysis [ABSTRACT FROM AUTHOR]