The extreme Northeast China cold vortex activities in the late spring of 2021 and possible causes involved

Abstract

Record-breaking numbers of Northeast China cold vortex (NCCV) occurred during the late spring (April–May) of 2021, which provided favorable background for more severe convection weather (such as hailstorm and tornado) happened and struck Jiangsu and Hubei provinces, China, causing heavy casualties and property losses. To better understand the possible causes of extremely abnormal NCCV activities, the external forcing and dynamical analysis was conducted. The results show that the extreme NCCV activity is regulated by the preceding sea surface temperature (SST) anomalies in the tropical Pacific, the snow conditions over the Tibetan Plateau, and the wave-mean flow interaction over the Eurasian continent. During the preceding autumn and winter in 2020, a moderate La Niña event occurred over the tropical Pacific, which triggered the Pacific–North America teleconnection pattern (PNA) like wave train and further dispersed the Rossby wave energy downstream along the mid-latitude westerly jet, forming a zonal wave train over the Eurasian continent. Moreover, the second minimum snow depth was recorded over the southeastern Tibetan Plateau during the boreal winter of 2020/2021, which induced a concurrent local anomalous anticyclone and a cyclone over northeast Asia in the following spring. Finally, the anomalous circulation is capable of achieving energy from the mean state through barotropic energy conversion and strengthening the downstream wave train accordingly. This study highlights the joint impacts of external forcings and internal atmospheric processes on the NCCV activity.