Research and Prediction of a Typical Stratospheric Sudden Warming Event in the Arctic in 2021

Taking the Northern Hemisphere stratospheric sudden warming (SSW) event in January 2021 as an example, which may considerably impact the winter cold wave event in China, this study discusses the characteristics and mechanism of the SSW in January 2021 using observation data, reanalysis data, and a prediction model. During this SSW event (hereinafter referred to as “event”), the Arctic stratospheric temperature increased by approximately 30 K in a few days, and westerly winds weakened over the Arctic and even reversed to the easterly winds thrice. In the early stage of the event, the Pacific sea surface temperature (SST) and Barents–Kara sea ice decreased abnormally. By examining the anomalies of an Eliassen–Palm flux, it is observed that these decreases result in more tropospheric planetary waves being propagated into the stratosphere; moreover, the increase in planetary wave activity in the stratosphere caused by a decrease in SST and sea ice may be the main factor responsible for the SSW event. Additionally, it is observed that during the event, the positive anomaly of the stratospheric geopotential height propagated downward into the troposphere, which is considerably related to the extreme cold event on the Eurasian continent in mid-January 2021. Finally, the stratospheric prediction model based on the WACCM6 (Whole Atmosphere Community Climate Model 6) and the DART (Data Assimilation Research TestBed) assimilation tool is used to study this event. The model successfully predicted the SSW event in January 2021. The predicted temperature and zonal wind intensity are essentially consistent with the observation results. Furthermore, the simulation results show that the assimilation of stratospheric temperature, ozone, and water vapor is crucial for accurately predicting the SSW event by reducing the errors of stratospheric circulation and temperature in the initial prediction field. However, when not assimilating stratospheric temperature, ozone, and water vapor in the initial field, the simulation results are considerably different from the observation; sometimes, they are even completely opposite.