Characteristics of Circulation Anomalies in Extended Range of the Persistent Extreme Cold Events in Northern Xinjiang during Winter

In this paper, daily station observational datasets from the National Climate Center and NCEP/NCAR reanalysis datasets during 1951–2019 were used to analyze the temporal and spatial distribution of persistent extremely cold events (PECE) in northern Xinjiang and to discuss the role of the stratosphere and troposphere in the occurrence and development of PECE. The results show that on an interdecadal scale, PECE frequency in northern Xinjiang peaked in the 1960s and gradually decreased since then, indicating that PECE frequency in northern Xinjiang is decreasing. In terms of spatial distribution, the centers of the PECE frequency and cold air intensity were located along the Irtysh River Basin. Stratospheric and tropospheric circulations were adjusted during PECE development. In the extension stage, 25 days before the event outbreak, the polar vortex (30°E–120°E) in the stratosphere began to change from strong to weak (Eurasian weak polar vortex type). Additionally, the planetary wave was dispersed downstream, and the southeastern branch wave train carried energy and affected the northern part of Xinjiang. Furthermore, the tropospheric polar vortex showed signs of weakening, with cold air splitting southwards. Subsequently, it entered a temporary stage. Cold air from the Arctic accumulated in front of the high-pressure ridge of the Ural Mountains and was conducive to the strengthening and maintenance of large inclined ridges. Meanwhile, cold air at the surface accumulated in Central Siberia. Finally, accompanied by strongly developed Siberian High, large-tropospheric troughs and inclined ridges led cold air to the south, which affected northern Xinjiang three days later.