Impacts of Sudden Stratospheric Warming on Extreme Cold Events in Early 2021: An Ensemble-Based Sensitivity Analysis

Impacts of sudden stratospheric warming (SSW) on three severe cold events (C1-C3) over Northern Hemisphere in early 2021 were investigated using subseasonal-to-seasonal ensemble forecasts. The SSW was characterized by successive displacement and split of the stratospheric polar vortex (SPV). Ensemble-based sensitivity analyses showed that C1 over Siberia and C2 over western Canada were more related to SPV variations at a lead time of 1-2 weeks than C3 over central U.S. Within ensemble forecasts, a more elongated SPV with higher geopotential height over northern Eurasia were conducive to C1, whereas a SPV with the more poleward retreat from displacement contributed to C2. The forecast accuracy of stratospheric heights over the sensitive region at 2 weeks lead was significantly correlated with the forecast skill of the tropospheric circulation pattern preceding C1, implying that prediction of specific cold weather events may be improved by a better forecast of key features in SPV variations.