中国北方干旱化年代际特征与大气环流的关系

Inter-decadal Characteristics of Aridification over Northern China Associated with the Variation of Atmospheric Circulation During the Past 50 Years

用CRU和ECMWF资料分析了近代中国北方干湿变化特征及其与东亚大气环流异常特征的关系。结果表明:中国北方干旱化具有显著的年际、年代际特征,20世纪70年代末干湿发生显著转变,西北东部和华北地区变干趋势明显,北方大部分地区干旱现象严重;中国北方地区当前的干旱化时空格局与东亚夏季风异常特征密切相关,夏季风减弱以及由此造成水汽输送量减少是导致干旱化发展的主要原因,而低层大气反气旋环流增强和气旋性环流减弱是引起干旱化的异常环流特征。

Aridification over northern China is currently one of the most urgent living environmental issues in our country. A lot of studies have been carried out concerning its evolution, causes and prediction, etc. There are different general circulation patterns at each dry/wet stages undoubtedly. In other words, an abrupt adjustment of summer monsoon circulation over East Asia will result in a distinct change of the precipitation pattern. However, up to now, analyses on the relationship between northern China drought and the summer south wind have been very limited. In this paper, inter-decadal characteristics of aridification over northern China and its association with the variation of atmospheric circulation during the past half century are analyzed based on the latest ECMWF and CRU reanalyzed monthly grid data. To better describe the detailed temporal and spatial features of droughts in northern China, the northern China is divided into 4 sub-regions and the annual Palmer Drought Severity Index （PDSI） is calculated in every unit respectively. It is shown that the annual precipitation and PDSI have decreased significantly in sub-regions B and C over the past 50 years. While in sub-region A, the annual precipitation and PDSI have an increasing trend from the end of 1950s to the mid 1990s followed by a decreasing period. In sub-region D, the annual precipitation and PDSI have a significantly decreasing trend from the end of 1950s to the end of 1970s, and increasing trend from the beginning of 1950s to mid 1990s followed by a decreasing period. As a matter of fact, it should be pointed out that the results in sub-region A may not be reliable enough as Wei Jie et al. suggested that PDSI is not suitable for extreme drought and wet regions. Therefore, the physical mechanism of drought in sub-regions B and C is mainly discussed with fo- cusing on the inter-decadal adjustment of summer monsoon circulation over East Asia. Multiple data analyses reflect the evident climatic jump at the end of 1970s over northern China. Accordingly, an inter-comparison of summer south wind, water-vapor transport and the 850 hPa anticyclone intensity between two periods is conducted. It is indicated that the south wind had experienced a strong period from 1958 to 1976 and then a weak period from 1977 to present. As a result, the precipitation distribution over northern China during the concemed periods altered obviously due to changes of the water vapor supply. Analyses on the 850 hPa anticyclone intensity present a similar conclusion. All these studies show that the aridification over northern China results from multiple factors： the weakened East Asian summer monsoon, the reduced water-vapor transport, and the increased anticyclone intensity, and so on.