长江中下游冬季两类冷空气活动事件的环流差异及其与急流的关系

利用1981—2020年台站观测的降水和最低气温资料,从长江中下游地区冬季冷空气活动事件中分出"降温有降水"和"降温无降水"两类事件(简称为"冷湿"和"冷干"事件),结合再分析资料,分析两类事件的冷暖空气活动特征和大气环流差异,进而探讨其与急流的联系。结果发现,长江中下游冷空气活动事件发生频次逐年减少,其中冷干事件发生频次逐年减少,冷湿事件发生频次显著增加,且冷湿事件的降温幅度大于冷干事件。冷湿事件发生时,高层副热带急流北移、极锋急流偏弱,中层存在倾斜槽脊、副热带高压偏西,低层西伯利亚高压增强,有来自西南的暖湿气流在长江中下游地区辐合上升;冷干事件发生时,高层副热带急流偏弱、极锋急流偏强,中层槽脊倾斜程度偏弱、副热带高压偏东,低层西伯利亚高压南移,有来自北方的偏干气流在长江中下游地区辐散下沉。此外,与冷湿事件相比,冷干事件过程中急流强度指数和经向风位置指数的波动更加频繁,冷空气活动持续时间更短。

Circulation differences between two types of winter 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 were 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. 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.