关键词: 西北地区, 干旱特征, 影响因子

Abstract: The humidity index was calculated based on the data of precipitation and evaporation in hot season (July to August) of 1971—2000 from the 140 meteorological observation stations in Gansu, Ningxia, Qinghai and Xinjiang. The space-time distribution of hot season drought was analyzed by method of REOF, from which five representative drought years and wet years were determined. The NCEP/NCAR reanalyzed information was used to compare the anomaly fields of Height at 500 hPa, Humidity at 700 hPa, SST and OLR between drought year and wet year. The following results have been obtained: The drought events happened in phase over the northwest of China, and the abnormal drought centers located in the east part of the northwest China, in the northwest of Qinghai, in Hexi Corridor of Gansu province, in southern Xinjiang and northern Xinjiang. The climate was remarkably arid in early 1970s and late 1990s, but more humid in 1980s. In drought years, the air circumfluence was high in the west and low in the east of northwest of China, cold air activated in Siberia, high ridge in Xinjiang was strong; the humidity anomaly field of 700 hPa was negative in the middle of Qinghai-Xizang Plateau and the west Asia and the west Pacific; the SST anomaly field was negative in the north and the west of Pacific, and positive in the equatorial region of the Pacific; the OLR anomaly field was positive in the north of China, the Indian peninsula, and the west of Pacific, especially the positive anomaly in Indian peninsula was higher than that of wet years. In wet years, the height anomaly field of 500 hPa was positive in the west and negative in east of Eurasia; the high ridge was prosperous in Ural, the trough in East Asia was deep; the humidity field of 700 hPa was positive in China continent, and the largest humidity center located in Qinghai-Xizang Plateau; the SST anomaly field was positive in the north and the west of the Pacific, and negative in Japan and the central and the equator of the Pacific; the OLR anomaly was predominately negative in China, and the center located in the central of Gansu and the south of Xinjiang.

Key words: northwest China, drought character, influencing factor