Based on the daily minimum temperature observation data in southeast of Gansu Province during 1969-2018, the variation of extreme low temperature events was analyzed, then 74 circulation characteristic quantities from National Climate Center were used to research the circulation system affecting the extreme low temperature events. The results are as follows: (1) The frequency of annual extreme low temperature events in southeast of Gansu Province reduced obviously with a rate of 2.3 d·(10 a)-1 in the last 50 years, and the reduction was most significant in summer and slowest in winter. There was an abrupt change of extreme low temperature days in 1987, after the abrupt change, the frequency of extreme low temperature events was relatively lower. (2) Compared with the climatic mean, the intensity of extreme low temperature events tended to increase, before 1987, the low temperature intensity anomaly increased with a rate of 0.2 ℃·(10 a)-1, while after 1987, the extrem low temperature intensity anomaly increased with a rate of 1.2 ℃·(10 a)-1. (3) The intensity of extreme low temperature in spring, summer, autumn and winter mainly ranged from -5.0～5.0 ℃, 10.0～15.0 ℃, -5.0～10.0 ℃, -20.0～-10.0 ℃, and the occurrence frequencies were 61.9%, 90.1%, 73.4% and 73.1%, respectively. (4) There was a positive correlation between extreme low temperature events and Eurasian meridional circulation in southeast of Gansu Province. The extreme low temperature events was related with cold air, the area index of western Pacific subtropical high and the intensity of polar vorticity center in the northern hemisphere in winter, while it was related with subtropical high northern boundary of South China sea, subtropical high northern boundary of the western Pacific and polar vorticity intensity of the Pacific in spring. The composited analysis of geopotential height fields indicated that Mongolia was a cold high pressure center on ground, and polar vorticity center was located in the eastern hemisphere from 500 hPa to 100 hPa, the westerly belt prevailed meridional circulation, and east Asian large trough in the westerly belt was deep and westward, the region of southeast of Gansu was controlled by strong northwest airflow after the trough, which was favourable to forming extreme low temperature events.

Based on the monthly precipitation data from 57 weather stations in the Loess Plateau from 1961 to 2015, the standardized precipitation index (SPI) was calculated as the index of drought and flood to study the circulation characteristics in abnormal drought and flood years in April in the Loess Plateau and their earlier forcing signals. The results are as follows: (1) The ridge from Ural Mountains to Xinjiang in China was stronger, and East Asian trough was deeper on 500 hPa, and the south wind was weaker on 700 hPa over the Loess Plateau and its east, and the temperature was  lower on 700 hPa from east China to Japan, and the westerly jet from east China to the ocean on the east of Japan moved southerly  and  became stronger. For these reasons, it became drought in April in the Loess Plateau, vice versa. (2) The frequency of the blocking in April in the range of 180°E-150°W in mid-high latitude of the Northern Hemisphere was more in the abnormal flood years than in the abnormal drought years. (3) The south Indian ocean dipole (SIOD) or the southwest Indian ocean SST could be used as a previous forcing signal for drought and flood in April. There was a significant negative correlation between SPI in April and SIOD of December in the previous years. (4) The responses of the circulation field to the positive and negative phase anomalies of SIOD were mainly reflected in the differences of the Ural high pressure ridge, the East Asian trough on 500 hPa and the circulations over the area from Lake Baikal to northeast China and Ural Mountains on 700 hPa.