关键词: 中国西北地区, SPEI干旱指数, 非线性特征, 经验正交函数, 近似熵

Abstract: From the view of climate system complexity, the nonlinear characteristics of drought were discussed in Northwest China from 1962 to 2017. Based on the monthly meteorological observation data at 159 stations of Northwest China, the standardized precipitation evapotranspiration index (SPEI) was calculated, firstly. Then, the temporal and spatial distribution of drought was analyzed by using empirical orthogonal function (EOF). And on this basis the abrupt change of decomposed time coefficient of SPEI was detected by using moving cut data- approximate entropy (MC-ApEn) method. Finally, the complexity of drought was discussed by using approximate entropy (ApEn) method in Northwest China. The results are as follows: (1) The first EOF mode of SPEI was consistently positive in Northwest China from 1962 to 2017, while the second EOF mode was positive in the east and negative in the west of Northwest China. Compared with time coefficient of the second mode, the climate was wetter in the west and drier in the east than that before the abrupt change, and the change point was approximately in 1980. The drought existed 2-3 a period in Northwest China during 1962-2017. (2) The complexity of drought increased after the abrupt change of 1980 in Northwest China as the whole, so the predictability of drought decreased. Moreover, the complexity of drought in autumn and winter was greater than that in spring and summer in Northwest China from 1962 to 2017, and the complexity was the largest in winter and the smallest in summer.

Key words: Northwest China, standardized precipitation evapotranspiration index, nonlinear characteristics, empirical orthogonal function, approximate entropy