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.

Automated summary

This study investigated the impacts of sudden stratospheric warming (SSW) on three severe cold events in the Northern Hemisphere in early 2021 using subseasonal-to-seasonal ensemble forecasts. The results showed that the stratospheric polar vortex (SPV) played a significant role in the occurrence of these cold events. The sensitivity analysis revealed that the variations in the SPV at a lead time of 1-2 weeks were more closely related to the cold events over Siberia and western Canada than the one over central U.S. The forecasts also indicated that the specific characteristics of the SPV, such as elongation and displacement, influenced the occurrence of these cold events.