Abstract: By using the NCEP/NCAR reanalysis datasets during 1974-2013, East Asian Monsoon indexes defined by Qiao Yunting are calculated and changes of the indexes are studied. The effect of East Asian Summer Monsoon on summer rainstorm days in Gansu is analyzed by using daily precipitation data at 80 stations in Gansu Province. The results show that the southwest monsoon, the southeast monsoon and the north monsoon are obviously inter-annual varied and different in summer. The southwest monsoon, the southeast monsoon has different effect on summer rainstorm in Gansu. When the prevalence of southwest monsoon, East Asian Trough is located from the Lake Baikal to the northwest China, and cold air and warm wet air from the Western Pacific subtropical high joins together at the southeast of Gansu. The region from the Lake Baikal to the Zhangye is occupied by southwest warm-wet air and northwest cold dry air, the water vapor from the Bengal Gulf meets in the southeast of Gansu, which causes rainstorm in the southeast of Gansu, the eastern Tianshui, the Pingliang, the western Qingyang and the Zhangye. When the prevalence of southeast monsoon, the western Pacific subtropical high extends westwards and northwards, cold air and warm air joins together at the mid-west of Gansu, the southwest water vapor from the Bengal Gulf meets in the southwest of Hedong, which causes the rainstorm in the Gannan Plateau, the middle of Guansu, Wuwei and Jiuquan.

Key words: East Summer Monsoon, interannual variability, atmospheric circulation, summer rainstorm days