2018年1月南方雨雪天气的形成及其与冬季风异常的关系

2018年1月3～7日和24～28日,我国南方地区出现了两次雨雪冰冻事件。本文利用台站观测资料和再分析资料,研究了它们的形成原因及其与冬季风异常的关系。结果表明,两次降水过程中,乌拉尔山地区有高压脊稳定维持,西风带多小波动活动,南支扰动活跃。受冷空气和南支槽等系统的影响,南方地区出现大范围降水、降雪,而这种稳定形势与平流层的环流特征有关系。1月平均的平流层极涡强度异常偏强,有利于低层环流的稳定。在1月份,南北不同纬带冬季风的强度有着显著的差别,大多数情况下呈现了不一致甚至相反的态势。南方雨雪冰冻天气往往发生在北方北风偏强、而南方南风偏强的时候。在季节尺度上,当冬季风呈现“北强南弱”的反相变化模态时,即北方北风、南方南风距平时,南方地区容易出现雨雪天气。

Formation of Rainy and Snowy Weather in South China in January 2018 and Its Relationship with the Abnormal East Asian Winter Monsoon

During the periods January 3rd to January 7th and January 24 to January 28, 2018, there were two rain and snow weather events over South China. The formative mechanisms for these storms and their relationship with an abnormal East Asian Winter Monsoon (EAWM) were studied using station observations and NCEP/NCAR reanalysis datasets. The results showed the mid-tropospheric circulation leading to these weather processes featured a persistent and stable ridge around the Ural Mountains, and active perturbations along both the mid-latitude westerlies and southern branch of the subtropical jet (SBJ). Due to the effects of cold air and the active perturbations along the SBJ, South China experienced extensive precipitation and snow. Simultaneously, there appeared an anomalously strong polar vortex in the stratosphere, which favors persistent and stable circulation systems in the troposphere. In January of 2018, the strength of the EAWM in the south and north were different. Most of the time, variations in the low-latitude and mid-high-latitude components of EAWM were inconsistent or even of opposite signs. Rainy and snowy weather in South China tends to occur when the EAWM presents itself in an opposing variation mode, i.e., a pattern of anomalous northerly winds to the north against southerly to the south. The relationship between the precipitation in South China and the opposing variation mode of the EAWM are also discussed on a seasonal scale, with the results confirming the above conclusions.