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.