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Abstract:
During sudden stratospheric warmings (SSWs), upward propagating planetary waves dissipate in the stratosphere and lead to a major deceleration of the climatological westerlies of the polar vortex. Events are often classified according to the dominant zonal wavenumber of the disturbance (displacement-type for zonal wavenumber 1 and split-type for wavenumber 2). To what extent anomalous wave forcing from the troposphere is involved in the formation of SSWs has been subject to extensive research, but is difficult to assess due to the observational sample size being small and the idealized nature of model experiments. This study utilizes extended-range ensemble forecasts from the S2S database to examine the role of tropospheric planetary waves before, during and after the 6101 predicted SSWs that are identified in these forecasts.First, we quantify anomalies in tropospheric wave activity ahead of SSWs, separately for split and displacement events. On average, prior to SSWs, the climatological zonal wavenumbers 1 and 2 strengthen, associated with enhanced vertical wave propagation. Locally, these anomalies constructively interfere over Eurasia, manifesting in high pressure over the Ural mountain domain, which has previously been identified as a potential precursor to SSW events. For example, we find that following high pressure over the Ural mountains, SSWs become about twice as likely compared to climatology.Second, we show that tropospheric features that contribute to the formation of SSWs, like Ural blocking, can be amplified through the induced stratospheric anomalies, which affects the surface response in the aftermath of the SSW event.
