The Drought Characteristics And Its Mechanism In The Main Arid Regions Of The Globe In The Background Of Global Warming

In this study, the monthly grid average air temperature and precipitation data created by Climatic Research Unit (CRU) of East Anglia University in England, and dry-wet index were employed to analyze the drought characteristics over the past100years. Furthermore, the drought area variation of the main arid region was studied on basis of these analyses. Finally, the large scale background of the main arid region drought was investigated from atmospheric general circulation and interaction between ocean and atmosphere. We draw the following conclusions:(1) In the last century, the temperature variability at different precipitation levels in globe and continents is greater than0, but the rate of temperature increasing was various at different precipitation levels and different arid regions. Global average temperature has increased about0.94℃/100a in arid regions, which was higher than the global warming of0.74℃/100a. The rates of temperature increasing in other arid regions were higher than the global warming except the South American mainland. That as, it was up to1.22℃/100a in the arid region in the North American continent, and to0.31℃/100a in the arid regions in the South American mainland.(2) It was showed that the precipitation of the global main arid regions has remarkable interdecadal changes, and the change trends were similar. Two obvious periods were shown in the arid regions of Australian, South-West Asia and Mongolia, and three obvious periods were shown in the arid regions of North America, North Africa and Central Asia.(3) Compared to the precipitation, the characteristics of dry-wet index also shown the obvious interdecadal changes, but the trends were not same as the precipitation changes. The periods of the dry-wet index changes were same as precipitation in the arid regions of South America, Australian, North Africa, South-West Asia and Mongolia, but the amplitudes of the changes were different. In addition, the precipitation was increasing in the arid regions of South Africa, North America and Central Asia with dry-wet index was reduced after the late1970s, which meant the trend was drier. (4) The percentages of drought area were different when it was evaluated by different indexes. The areas of arid regions were increasing when it was evaluated by dry-wet index, the drought areas were near up to30%of the globe after the late1970s. The drought area amplitudes were most evident in arid region of African continent, then the Australia, followed by Asia, and finally the South and North American. Similarly, the percentages of drought areas were obviously different when it was evaluated by precipitation. Especially, it was shown the opposite trend after the late1970s because of the significant warming effect.(5) The drought of the main arid regions was closely related to the abnormality of atmospheric general circulation. In the typical precipitation months of the drought year in northern and southern hemisphere, the anomaly of monthly mean circulation fields of500hPa was positive. This kind of abnormality of circulation made the Hadley circulation sink which increased the drought of the arid regions of the South America, South Africa, Australia, North America, North Africa and Southwest Asia, respectively, this kind of abnormality of circulation enhanced the downdraft of thermal compensation which caused by the Tibetan plateau terrain in Central Asia and Mongolia, and the drought was increased.(6) In the decadal scale, the drought of the main arid regions was closely related to the sea surface temperature. The Pacific Decadal Oscillation (PDO) index was jumped between1960s and1980s, and1976was the turning point. The trend of dry-wet and PDO index was same in arid regions of South and North America with opposite in arid regions of South Africa, Southwest Asia and the Mongolia; the trend of dry-wet index and PDO index in North Africa was same before1976, after that the trend was opposite; the trend of dry-wet index and PDO index in Australia and Central Asia was independent. The obvious trend of the North Atlantic Oscillation (NAO) year was appeared after1950. During this period, the trend of dry-wet index and NAO was same in South and North America with opposite in South and North Africa and Southwest Asia; the trend of dry-wet index and NAO index in Australia and Central Asia and Mongolia was independent. The Southern Oscillation (SO) index had no remarkable change period during last century.(7) In the decadal scale, the PDO and SO indexes were the same importance to dry/wet changes in globe, but the signs of the correlation coefficients were opposite. The correlation coefficients of dry-wet and NAO indexes which passed the significance of95%were scattered. The dry-wet and PDO indexes were positive correlation in South America, the dry-wet and SO indexes were positive correlation in South Africa and Australia. The dry-wet index was positive with PDO and NAO indexes, but was negative with SO index. The dry-wet index was obviously negative with PDO in North Africa and Southwest Asia, but was positive with NAO in Center Asia. The dry-wet index was positive with PDO and NAO indexes in Mongolia.