中文摘要：

此系姐妹篇文章的上篇，利用中蒙地区106站19542005年间的实测降水资料，使用EOF及REOF等统计方法，整体地分析了中蒙干旱半干旱区降水的时空变化特征。结果表明：（1）中蒙干旱区的降水分布很不均匀（我国西北区尤甚），从该区东南侧最湿的陕南向南疆盆地东端极端干旱区过渡，年雨量从922mm锐减到25mm以下，最干的托克逊仅6．9mm；蒙古国以中北部最湿，年雨量达400mm以上，到南部仅100mm左右，再到西南角仅30mm左右。（2）该区降水分布受不同尺度地形影响大，降水的年及年际变化也大。约80％的降水集中在夏半年（5～9月），南疆盆地东端干中心区的年降水相对变率最大，接近0．80，远高于蒙古国和我国其他地区，也高于全球明显增暖前。其干旱程度及降水变率居世界8大干旱区前列。（3）该区降水异常分布的局域特征明显。区内夏半年的降水异常分布常出现河东、南疆、汉中关中、北疆、蒙古国中北部、河西阿拉善，以及柴达木等7个模态。（4）过去50余年来，中蒙地区东北部的降水总体呈线性减少趋势，而西南部则呈增加趋势。

英文摘要：

To better explore the temporal and spatial change features of the precipitation over China- Mongolia Arid-and Semiarid-Areas（ hereafter CMASA）, in the Part I of the companion papers, the annual precipitation feature and the May- September one REOF analyses over the CMASA have been, as a whole, conducted, utilizing the observed precipitation data at 106 stations totally over the CMASA during the period from 1954 to 2005 and the EOF and REOF techniques. The main conclusions are as fol- lows： （1） The main body and dry center over the CMASA are in Northwest China（NWC）. The precipitation distribution is very uneven over CMASA, particularly in NWC. On the China side, the most wettest region over the CMASA is in South Shaanxi, with the annual mean rainfall of 922 mm, but with the transition from there to the east end of Tarim Basin , the precipitation reduce sharply. At Toksun, the driest core over CMASA with the multiyear mean rainfall of only 6.9 mm. On the Mongolia side, the Central-and Northern-Mongolia is something wet, with the mean rainfall of more than 400 mm; but in South Mongolia Gobi, around 100 mm; and again in the driest southwestern Gobi Altai, only 30 mm or so. （2） There is heavily impact of the topography on the different scales like the Qinghai-Xizang Plateau（Plateau）, Tianshan, Hangai Shan, even the Dabacheng Gorge on the precipitation distribution over the CMASA. （3）There are largely seasonal, interannual and interdecadal variations of the precipitation over the CMASA. About 80% of the precipitation is concentrated from May to September. The relative variation rate of interannual precipitation near the dry center is as 0.78 as high. （4） According to REOF analyses of precipitation for May- September over CMASA, the precipitation anomaly distribution often have seven modes： region on the east side of Yellow River, South Xinjiang, Gansu Corridor and Alashan, Hanzhong and Guanzhong, North Xinjiang, Central and North Mongolia, and Qaidam Basin. （5）In the past 52 years, o