2018年和2008年1～2月影响我国的持续低温事件及其对比分析

2018年1月中下旬至2月初我国发生了一次大范围的持续低温异常事件，给国民经济和人们生活带来了很大的不利影响。这次持续低温事件与众所周知的2008年1月发生在我国南方的持续雨雪冰冻事件（简称“0801”事件）有一个共同点，二者都是发生在La Ni?a和前秋北极海冰显著减少的背景之下。对比这两次事件发生的大气环流和热力背景，对认识冬季持续极寒事件的形成有重要意义。本文利用NCEP/NCAR大气环流再分析资料和NOAA卫星观测OLR资料，对这两次过程进行了比较。结果显示，与“0801”事件相比，2018年1月中下旬的低温事件表现出以下明显不同的特征:（1）过程持续短，冷空气路径偏东；（2）乌拉尔阻塞和东亚大槽并存，呈“西高东低”型，不同于“0801”高纬阻塞和低纬南支槽并存的“北高南低”型；（3）异常的环流型与自大西洋经乌拉尔山至东亚传播的波列有关，西伯利亚高压持续时间短；（4）热带对流活跃在海洋性大陆和西太平洋，不同于“0801”主要在印度洋。后者对该年南支槽和西太平洋副热带高压增强有显著贡献。最后，2018年过程期间水汽输送也不如“0801”强，导致降水弱。

Comparison of Persistent Cold Events in China during January–February of 2018 and 2008

A persistent cold event affected China through late January to early February 2018, causing severe damage to the life and property of the country. The event exhibits a resemblance to the well-known long-lasting snowstorms of January 2008 (“0801” event in brief), because both the events occurred during the La Ni?a phase along with reduced sea ice in the Arctic during the preceding autumn. In this study, the observational features of the 2018 event and the possible factors for its occurrence are investigated through a comparison with the “0801” event. The 2018 cold event persisted for a relatively shorter period than the “0801” event, with cold air outbreak along with an eastern pathway rather than a northwestern or northern pathway, as was seen in “0801”. Large-scale atmospheric circulation exhibited a substantial difference between the two cases. The Ural blocking and the East Asian grand trough, the two key systems consisting of East Asian winter monsoon (EAWM), which formed a “west-high-east-low” pattern, were significantly stronger in the 2018 event. In comparison, the East Asian grand trough in the “0801” event did not deepen as evidently, despite the similarly enhanced Ural blocking. Instead, a substantially deepened southern branch trough, another important composition of EAWM, developed in “0801” and resulted in a “north-high-south-low” pattern. Also, the Siberian high persisted for a shorter period in the 2018 event. Besides, tropical convection activities exhibited difference between the two cases. Enhanced convection occurred in the Maritime Continent and equatorial western Pacific in the 2018 case, but mainly over the equatorial Indian Ocean during the “0801” event. The latter contributed to the strengthening of the southern branch trough and the western Pacific subtropical high leading to intensified water vapor transportation from the Bay of Bengal and the western Pacific, and intensified precipitation. In comparison, the convection in 2018 was substantially weaker and not so favorable for transporting moisture to South China.