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Sudden stratospheric warmings (SSWs) are the most dramatic wintertime stratospheric phenomena. They are preceded by a sustained wave dissipation in the stratosphere that leads to the deceleration of the polar vortex. The signal from SSWs then typically propagates downward reaching the troposphere and inducing a negative phase of the Annular Mode that may persist several weeks up to two months. Incorporating then stratospheric information in subseasonal to seasonal (S2S) forecast systems has been shown to improve the skill of S2S predictions for surface climate. However, on average, present S2S forecast systems can only predict SSWs around two weeks before the onset of the event. A suggested strategy to increase their predictability is to improve the representation of triggering mechanisms of SSWs. However, while there is a consensus on the relevance of the wave activity for that, the origin of the rapid enhancement of stratospheric wave activity prior to SSWs is not sufficiently understood.The aim of this study is two-fold: to assess the ability of forecast systems to reproduce the stratospheric wave amplification during SSWs and to quantify the role of the stratosphere in this enhanced upward wave propagation. To do so, we analyze the triggering mechanisms of three different SSWs, the boreal SSWs of 2018 and 2019 and the austral minor SSW of 2019, by means of SNAPSI (Stratospheric Nudging And Predictable Surface Impacts) sets of forecast ensembles. These ensembles include free-evolving atmospheric runs and nudged simulations where the zonally-symmetric stratospheric state is nudged to either observations of a certain SSW or a climatological state. Our results show that models struggle to predict the SSW of 2018, as they are not able to capture the strong enhancement of wavenumber-2 wave activity around one week before the event. In contrast, most ensemble members of all models are able to simulate both SSWs of 2019, but with some common issues such as an early timing for the NH event and a weaker deceleration of the vortex in the case of the SH SSW. In the three cases, capturing both the tropospheric precursors and the interactions of waves with the stratospheric flow are revealed to be crucial for the occurrence of the phenomena. However, the relative role of each contribution is different depending on the individual event. This is a contribution of the Working Group 4 of the SNAPSI initiative.
Ayarzagüena et al. (Fri,) studied this question.