The variation in surface wetness index (SWI), which was derived from global gridded monthly precipi- tation and monthly mean surface air temperature datasets of Climatic Research Unit (CRU), from 1951― 2002 over global land was analyzed in this paper. The characteristics of the SWI variation in global continents, such as North America, South America, Eurasia, Africa, and Australia, were compared. In addition, the correlation between the SWI variation of each continent (or across the globe) and the large-scale background closely related to SST variations, which affects climate change, was analyzed. The results indicate that the SWI variation shows distinct regional characteristics in the second half of the 20th century under global warming. A drying trend in the last 52 years occurred in Africa, Eurasia, Australia and South America, most obviously in Africa and Eurasia. North America shows a wetting trend after 1976. A 30-year period of dry-wet oscillation is found in South America and Australia; the latest is in a drying period in two regions. The results also revealed that global warming has changed the dry-wet pattern of the global land. South America and Australia have a drying trend despite in- creases in precipitation. This indicates that increases in surface air temperature cannot be ignored in aridification studies. Global dry-wet variation is closely related to large-scale SST variations: the drying trend in Africa and Eurasia and the wetting trend in North America are correlated with Pacific Decadal Oscillation (PDO); the interdecadal oscillation of SWI in South America and Australia is consistent with the interdecadal variation in Southern Oscillation Index (SOI).
利用CRU（Climatic Research Unit）的全球格点月降水及月平均气温资料，通过计算地表湿润指数分析了1951～2002年全球干湿变化趋势．重点对比研究了北美大陆、南美大陆、欧亚大陆、非洲大陆及澳洲大陆干湿变化的特征及差异，并在此基础上对各大陆乃至全球的干湿变化与影响气候变化的大尺度背景的相关关系进行了分析．结果表明，在增暖背景下，20世纪下半叶全球干湿变化趋势具有明显的区域差异，非洲大陆、欧亚大陆、澳洲大陆和南美大陆近52年主要以干旱化趋势为主，尤以非洲大陆和欧亚大陆最为剧烈．北美大陆在1976年以后表现为变湿趋势，南美大陆存在30年左右的干湿振荡周期，但最近仍然处于干化的时段，澳洲大陆的变化与之类似．研究还表明，增暖已经改变了全球环境干湿变化的分布格局，南美大陆和澳洲大陆尽管降水为增加趋势，但仍然表现为干旱化趋势，其中温度升高是其表现为干旱化特征不可忽视的原因．全球环境的干湿变化与大尺度背景密切相关：非洲大陆、欧亚大陆的干旱化趋势、北美大陆的湿化趋势与北太平洋年代际振荡（PDO）显著相关；而南美大陆、澳洲大陆干湿的年代际振荡与南方涛动指数（SOI）的年代际变化一致．
Based on monthly precipitation and monthly mean surface air temperature (SAT),the dry/wet trends and shift of the central part of North China and their relationship to the Pacific Decadal Oscillation (PDO) from 1951 to 2005 have been analyzed through calculating surface wetness index (SWI). The results indicate that there was a prominent drying trend and an abrupt change in the analysis period. A per-sistent warming period with less precipitation from the mid and late 1970s to present was found,and a shift process exists from the wet to the dry in the central part of North China during 1951-2005. The transition is located in the mid to late 1970s,which should be related to the shift variation of large-scale climate background. The correlation analysis has brought about a finding of significant correlativity between PDO index (PDOI) and SAT,precipitation and SWI in this region. The correlation exhibits that the positive phase of PDOI (warm PDO phase) matches warming,less precipitation and the drought period,and the negative PDOI phase corresponds to low SAT,more precipitation and the wet period. The duration of various phases is more than 25 years. The decadal variation of sea surface temperature (SST) in the North Pacific Ocean is one of the possible causes in forming the decadal dry/wet trend and shift of the central part of North China.
