An extreme cold event over East Asia during early January 2021 and its links to the deformation of stratospheric polar vortex during sudden stratospheric warming

In early winter of 2020/2021, East Asia experienced two extreme cold events (ECEs), during which a number of national stations in China reached the standard of ECE or broke records. The underlying dynamical processes of these record-breaking ECEs are investigated using Modern-Era Retrospective Analysis for Research and Applications version 2 reanalysis dataset. Results show that these ECEs were closely related to the sudden stratospheric warming (SSW) event. During 29 December, 2020-1 January, 2021 (the first ECE), the tropospheric polar vortex migrated to mid-latitude East Asia and more planetary wave propagated into the stratosphere. Then, the stratospheric polar vortex weakened and extended to mid-latitude East Asia from 2 January. And, a SSW event occurred on 5 January. The easterly zonal wind accompanied by the deformation of stratospheric polar vortex over East Asia tended to strengthen the East Asian trough via reflecting the planetary wave from the stratosphere to troposphere. Simultaneously, the southward shifting of the Ural ridge resulted in the extremely strong East Asian trough migrating southward, which led to the strengthening and southward migration of near-surface anticyclonic circulation. Then, another ECE broke out during 6-7 January (the second ECE). Therefore, the ECE in early January was jointly caused by the combined effects of deformation of stratospheric polar vortex during SSW event and the southward shifting of Ural ridge. Further analysis suggested that about 8% of the total ECEs during 1980/1981-2019/2020 were related to the extension of stratospheric polar vortex to East Asia.