Abstract: Using daily mean air temperature data of 25 meteorological observations in Ningxia, the space-time evolution of ≥0 ℃ accumulated temperature and its beginning and ending dates are statistically analyzed. The period and sudden change of ≥0 ℃ accumulated temperature are revealed by complex Morlet wavelet analysis and Mann-Kendall mutation detection methods, respectively. And the cause of its climatic evolution has been primarily discussed through comparable analysis of atmospheric circulation characters between before and after climate shift. The results show that: during the past 50 years, the ≥0 ℃ accumulated temperature in the whole area of Ningxia shows an increasing trend, especially in the background of climate warming the increasing speeds are the most, with averaged increase of 231.8 ℃. The beginning time of stably ≥0 ℃ accumulated temperature has been early and ending time late, so its duration has became significantly longer, especially in the latest 10 years, with 8 day longer averaged in all Ningxia. The ≥0 ℃ accumulated temperature in the whole Ningxia and in the area of Yellow River Irrigation Area of Ningxia changed suddenly in 1997 from less to more, and there were about 3-4 a and 9 a periods since 1960s. Among these periods the 9 a period is relatively stronger, and with the strongest in the 1990s. The days of extreme minimum temperature of <0 ℃ in the area of Yellow River Irrigation Area has been significantly reduced during the period of ≥0 ℃ accumulated temperature. So correspondingly the frost damages also should be reduced. And the related departments should adjust agriculture structure distribution in time according to the indices proposed in this paper. It is atmospheric circulation significant adjustment before and after climate warming that caused increasing trend and duration time longer of ≥0 ℃ accumulated temperature. Before climate warming change, in the highmid latitude region on 500 hPa height, especially in area near Balkhash Lake, it was easy to form low trough, so it was advantage of cold air in the polar going down southerly and affecting Ningxia, and causing temperature lower, and ≥0 ℃ accumulated temperature smaller. While after climate warming change, the atmospheric circulation on 500 hPa changed significantly. The meridional circulation has been weakened, and the latitudinal circulation strengthened. This was disadvantage of low trough forming and developing, the numbers of cold air going down southerly from polar reduced, the temperature raised in the largescale area of downstream, and so corresponding to the ≥0 ℃ accumulated temperature increase.

Key words: Ningxia, 0 ℃ accumulated temperature, spacetime evolution