Circulation differences between two types of winter extreme cold air activity events in the middle-lower reaches of the Yangtze River and their relationship with the jet stream

Based on the precipitation and minimum temperature data observed at the stations from 1981 to 2020, two types of winter cold air activity events in the middle-lower reaches of the Yangtze River are identified, which are strong cooling events with precipitation and only strong cooling events without precipitation (referred to as “cold-wet” and “cold-dry” events), respectively. The atmospheric circulation differences and the characteristics of cold and warm air activities in the two type events, and their connection with the jet stream are also examined. The results show that the frequency of cold air activity events in the middle-lower reaches of the Yangtze River decreases year by year, among which the frequency of cold-dry events decreases and the frequency of cold-wet events increases significantly. In addition, the cooling amplitude of cold-wet events is greater than that with no precipitation. When cold-wet events occur, the upper-level subtropical jet moves northward and the polar front jet is weaker. There is a tilting ridge in the middle level and the subtropical high is located westward. In the lower-level, the Siberian high is strengthened and the warm and humid air flowing from the southwest converges and ascends in the middle-lower reaches of the Yangtze River. However, when cold-dry events occur, the subtropical jet in the upper level is weaker and the polar front jet is stronger. The trough and ridge in the middle level are weaker and the subtropical high is located eastward. In the lower level, the Siberian high is located southward and the dry air flowing from the north diverges and sinks in the middle-lower reaches of the Yangtze River. In addition, compared with cold-wet events, the jet stream intensity index and the meridional wind position index fluctuate more frequently during cold-dry events, and the cold air activity duration is shorter.