Subseasonal to seasonal (s2s) prediction of continental cold following the sudden stratospheric warming in the 2022/23 winter

Using the ERA5 reanalysis and subseasonal to seasonal (S2S) real-time forecasts, this study explores the prediction of stratospheric disturbances in the 2022/23 winter, with a particular focus on the February 2023 major sudden stratospheric warming (SSW). Diagnostics of the annular mode index in the Northern Hemisphere (NAM) and the circulation anomalies reveals that stratosphere-troposphere coupling was enhanced in three periods and drove surface climate extremes. The February 2023 SSW was primarily driven by enhanced wave activities originating in the lower stratosphere. In the enhanced stratosphere-troposphere coupling periods, cold anomalies prevailed over Asian or/and North American midlatitudes, and compound wet-cold anomalies were evident in Northeastern Asia or/and North Atlantic. Snowstorm frequency increased along the midlatitude edge of the stratospheric and especially tropospheric polar vortices. Forecasts initialized around two lead time groups capture one SSW, one within the medium range time limit (1–2 weeks, true alarm), and the other (4–6 weeks, earlier alarm) far beyond the typical prediction skill of SSWs. During periods with strong stratosphere-troposphere coupling, the prediction skill of precipitation and snowstorms was higher with a more realistic spatial pattern than the weak coupling periods. Consistent with the reanalysis, the snowstorms were more active where the tropospheric polar vortex extended equatorward in models. However, models also tend to exaggerate the tropospheric impact of the SSW and forecast more snowstorms along the midlatitude vortex edge. Forecasts with a better SSW representation usually capture the cold over the North American continent during SSW occurrence and post-SSW periods.