Interdecadal fluctuation of dry and wet climate boundaries in China in the past 50 years

Based on the mean yearly precipitation and the total yearly evaporation data of 295 meteorological stations in China in 1951-1999, the aridity index is calculated in this paper. According to the aridity index, the climatic regions in China are classified into three types, namely, arid region, semi-arid region and humid region. Dry and wet climate boundaries in China fluctuate markedly and differentiate greatly in each region in the past 50 years. The fluctuation amplitudes are 20-400 km in Northeast China, 40-400 km in North China, 30-350 km in the eastern part of Northwest China and 40-370 km in Southwest China. Before the 1980s (including 1980), the climate tended to be dry in Northeast China and North China, to be wet in the eastern part of Northwest China and very wet in Southwest China. Since the 1990s there have been dry signs in Southwest China, the eastern part of Northwest China and North China. The climate becomes wetter in Northeast China. Semi-arid region is the transitional zone between humid and arid regions, the monsoon edge belt in China, and the susceptible region of environmental evolution. At the end of the 1960s dry and wet climate in China witnessed abrupt changes, changing wetness into dryness. Dry and wet climate boundaries show the fluctuation characteristics of the whole shifts and the opposite fluctuations of eastward, westward, southward and northward directions. The fluctuations of climatic boundaries and the dry and wet variations of climate have distinctive interdecadal features.

THE FLUCTUATIONS OF DRY AND WET CLIMATE BOUNDARY AND ITS CAUSAL ANALYSES IN CHINA

Based on yearly precipitation and Φ20 evaporation pan data during 1951 to 1999 of 295 stations,the aridity index is calculated in this paper.According to the aridity index,the climatic regions in China are divided into three types:the arid zone,the semi-arid zone and the humid zone. Isoline 0.20 is the boundary between arid and semi-arid zones.Isoline 0.50 is the boundary between semi-arid and humid zones.The fluctuations of dry and wet climate boundaries are very substantial,have greatly regional difference,and have the features of the whole shifting along the same direction and of the opposite moving along the contrary direction over the past 50 years.The semi-arid zone is a transitional zone between humid and arid zones,a border belt of monsoon,and a susceptible zone of environmental evolution in China. In the period of the late 1960s to the early 1970s,remarkable change had occurred for dry and wet climate in China.It manifests significantly that climate is from wetter into drought in most regions of northern China.Moreover,drought has an increasing trend.The fluctuations of climatic boundaries and the dry and wet variations in climate have substantial inter-decadal features. The main factors affecting the dry and wet climate boundary fluctuations and the dry and wet variations of climate in China are East Asian summer monsoon,Indian Monsoon,plateau monsoon in the Tibetan Plateau,westerly circulation,and West Pacific subtropical high.The different types of circulations and the strong and weak combinations of these circulations result in the regional differences of dry and wet climate changes in China.Inter-decadal variations of the dry and wet climate boundary fluctuations and of the arid and humid climate result from the inter-decadal changes of East Asian summer monsoon,Indian Monsoon,plateau monsoon,westerly circulation, and West Pacific subtropical high.The anomalous general atmospheric circulation in the Northern Hemisphere during the late 1960s to the early 1970s is the causes of arid and humid climate remarkable change in China.