Characteristics and circulation causes of cold wave events affecting different ranges in Jiangsu in winter

Based on the daily air temperature observation data and JRA55 reanalysis data from 69 stations in Jiangsu during 1979—2021, the spatio-temporal characteristics of cold wave events (CWE) affecting different ranges in Jiangsu in winter (November to March) and the causes of circulation are investigated. The results show that the number of the regional cold wave events (R-CWE) affecting Jiangsu in winter was obviously more than the large-range cold wave events (L-CWE). The areas affected by R-CWE were mainly concentrated in the Huaibei area, and the areas affected by L-CWE were located in the middle part of Huaibei and the southern part of Jianghuai. In addition, there were significantly more R-CWE in November, December and March than in January and February, and more L-CWE in November than in other months. The linear increase trend of R-CWE under global warming was more significant than that of L-CWE, especially the frequency of R-CWE increased significantly after 2010. The analysis of circulation shows that a pair of large oblique troughs and ridges near the middle and high latitudes of 500hPa in Eurasia were dominant on day -4 of L-CWE, and cold air in the area north of 75°N could continuously flow southward from the northwest path, while the intensity of circulation anomalies corresponding to R-CWE were significantly weaker than that of L-CWE in the same period, and cold air mainly came from the area south of 75°N. The strengthening degree of the circulation system near the Ural Mountains is the key to distinguishing the two types of CWE. On day -2, the intensity of the Siberian high at L-CWE reached 1043hPa, 5hPa larger than that of R-CWE. On day 0, its southern boundary extended to 31°N, while R-CWE was located near 38°N. Further investigation showed that the maximum instantaneous blocking frequency near the Ural Mountains during the occurrence of L-CWE was 46%, and the anomaly of the Siberian high pressure index was basically maintained from day -2 to day -1 and reached a peak of 6.54hPa on day -1, with an intensity about twice that of R-CWE. In the early stage of L-CWE, there was a Rossby wave train from the middle and high latitudes to the southeast and can be transmitted to the south of Jiangsu, which was quite favorable for the occurrence of a more widespread cold wave event in Jiangsu. In contrast, the Rossby wave train of R-CWE starts 1—2 days later and the intensity was weaker.