The Variation Characteristics Of The South Belt Boundary Of The East Asian Winter Monsoon And Their Impacts On Low Temperature, Snow And Freezing In Southern China

Based on monthly mean and four times a day ERA-interim data, NCEP/NCAR reanalysis data and observed data of 753 stations in China, the existence of the south boundary belt of East Asia winter monsoon is discussed by investigating on standard deviation and meridional gradient distribution characteristics of temperature, wind, relative humidity and pseudo-equivalent temperature on lower layer and temperature, snow cover days on surface. The southern boundary belt of East Asia winter monsoon is defined on the basis of the stations with and without snow days and its fluctuant features are studied. By using the threshold range and the duration of the meteorological elements, the comprehensive evaluation index of low temperature, snow and freezing in Southern China is established and he risk zoning is divided. On that case, the possible influence of the position of southern belt on low temperature, snow and freezing in Southern China is analyzed. The major conclusions are summarized as follows:(1) Both deviation and meridional gradient of temperature, wind, relative humidity and pseudo-equivalent temperature field arises high value regions in South China, and the salient regions are consistent with climatic mean line of the number of days without snow. The results indicate that the distribution characteristics of various meteorological elements are in accord with the climatic features of monsoon marginal belt, and can support the existence of the south boundary belt of East Asia winter monsoon. Thus, primarily conclude that East Asia winter monsoon south boundary belt is existed. The position varies basically form 24°N to 30°N, and has a northward trend during the past 30 years as a possible result of global warming. In the years that the southern boundary anomalous north, the west wind over 500hPa above Southern China is weak, the geopotential height is strong and the temperature in East Asia is overall high. Over 850hPa, there remain southerly wind anomalies, the geopotential height is low in north and high in south, which is not conducive to the southward advance of north wind. When the southern edge position is further south(north), polar vortex area is relatively large(small), and the subtropical high area is smaller(larger).(2) When low temperature, snow and freezing weather occurs, the southern regions usually under the conditions of low temperature, high humidity and scant sunlight. The light risk areas in Southern China mainly located in the northern Sichuan Basin and southern hill areas. The intermediate risk zone is surrounded by light risk areas, shaped like "U", rises from the western regions of the Sichuan Basin, goes through the northern part of Yunnan-Guizhou Plateau and the eastern part of the Yangtze River Basin, and extends to the southern areas of the Huang-Huai Plain and the Loess Plateau. Heavy risk areas include the central and southern regions of Sichuan Province, northeastern part of Yunnan Province, western areas in Guizhou Province, eastern ares in Hunan Province, northern regions of Jiangxi Province, Anhui Province, most areas in Hubei Province and some areas in Shanxi Province. As time progresses, due to the reduction of meeting days over temperature and humidity condition, the frequencies for the occurrence of light, intermediate, heavy and extra heavy low temperature, snow and freezing were decreased. However, in the late ten years, as the transform of the meteorological conditions for the formation of low temperature, snow and freezing weather, the risk became higher.(3) When the south edge of the East Asian Winter Monsoon locates further north (south), the number of low temperature, snow and freezing days and each level events are less (more). And in the years that the position of south edge locates north (south), the subtropical westerly jet is usually weaker (stronger), and the temperature in winter over Southern China is generally high (low). In the years that the position of south edge locates south, the meridional wind over Southern China is weaker, near zero value, and the air warmer air mass in south and colder air mass entrenched in north confronts in this area, causes the instability of flow, which means the formation rain and snow weather is more susceptible. In the retention period of the East Asian winter monsoon’s south edge, the subtropical westerly jet is strongest, the scale of the convergence area of southerly and northerly winds is largest, the meridional wind is weakest, and the number of days of low temperature, snow and freezing is most. In the advance period, the number of days is medium, and the withdrawal period at least.