Abstract: On the basis of ERA Interim reanalysis data and monthly precipitation observation station data of 160 stations over the eastern northwest China(32.5°N-42.5°N, 100°N-112.5°N)during 1979-2015, the regional climatic characteristics of water vapor transportation over eastern northwest China, characteristics of vertically integrated water vapor transportation corresponding with the EOF modes of precipitation, characteristics of geopotential height and water vapor transportation anomaly during strong(weak) precipitation years were revealed through empirical orthogonal function(EOF)analysis, composite analysis methods. The results show that the strongest summer latitudinal vapor transportation is at 700-500 hPa, and the strongest summer longitudinal vapor transportation is at 900-800 hPa over the eastern northwest China. The first EOF mode of eastern northwest China precipitation is consistent over the whole area. Corresponding to the first precipitation mode, there is a cyclonic anomaly to the east of Lake Baikal and an anticyclonic anomaly over south-east China and the water vapor transported by west wind almost prevails the whole area. Water vapor transported by wet south-west wind from prevails to the south of Monsoon Fringe. The second EOF mode of precipitation shows southeast-northwest pattern. Corresponding to the second EOF mode, an anticyclonic anomaly centers around Siberia and the westerly wind area is located to the north of the anticyclonic anomaly. Water vapor is mainly transported by easterly wind. The strongest precipitation area is around the east boundary because the water vapor transported through east boundary is reduced at the transportation process. There is a cyclonic anomaly around the Yellow Sea. Qinghai-Tibet Plateau and the monsoon fringe area are to the north-west of the cyclonic anomaly. Water vapor is mainly transported by north-east wind from the ocean to the east of China. The third mode shows northeast-southwest pattern. Corresponding to this mode, there is a cyclonic anomaly in the Northeast of China and the water vapor is transported to the east of Northwest China by westerly wind and north-westerly wind to the south of the cyclonic anomaly. The composite of geopotential height and water vapor fluxes between the strong precipitation years and weak precipitation years shows that an anticyclonic anomaly is over Southeast China, which benefits the south-west water vapor to come into the monsoon fringe. At the same time, there is a cyclonic anomaly to the east of Lake Baikal providing westerly and north-westerly water vapor transportation, and the convergence is strengthened as well. The circulation configuration is similar to the 500-hPa geopotentia height field and water vapor transportation field regressed to the first EOF mode of precipitation.

Key words: eastern northwest China, water vapor transport, EOF, composite analysis