Impact of tropospheric polar vortex on winter cold extremes over Northeast China

Abstract

Using CN05.1 dataset and NCEP reanalysis, this study investigated the potential linkage between the tropospheric polar vortex and extreme winter cold events over Northeast China. Results show that Asian part of polar vortex (APV) is expanded and weakened before the cold events, this pattern manifests itself as a significant correlation a week before anomalous cold events. Moreover, these cold events could be classified into two types according to the polar vortex position, i.e., eastward-shifted and westward-shifted polar vortex events (EPVE and WPVE). There are significant differences in tropospheric circulation between these two types of events from 14 to 7 days before the cold event outbreak. More specifically, blockings over western Europe and the Bering strait during early-middle stage of EPVE are favourable for the shift of polar vortex towards the northern Eurasia. The southward-shifted polar vortex enhances Rossby wave activity, favourable for the development of Ural blocking, cold vortex in East Asia and cold air transport. APV is greatly weakened after the onset of cold outbreak, leading to cold air intrusion from the western side of cold vortex into Northeast China. During WPVE, the polar vortex is located over the Baffin Island. APV and cold vortex are weaker before the cold event outbreak. The cold air is mainly induced by the blocking high ridge over East Siberian Sea, with a longer transport path and weaker intensity compared with that during the eastward events. In conclusion, the cooling over Northeast China is more drastic due to stronger cold air transport when the polar vortex centre is shifted towards Eurasia.