Spatial and temporal variability of drought during 1960–2012 in Inner Mongolia, north China

In this study, drought in Inner Mongolia was studied by using standardized precipitation index (SPI) and dry spells (DS). Principal component analysis (PCA) was applied to the SPI series computed on 24-month time scale. The results demonstrate noticeable spatial patterns, with six sub-regions characterized by different climatic variability. Overall, most parts of Inner Mongolia are dominated by dry tendencies, except for the western and easternmost areas. Three DS indices, including the number of dry spells (NDS), mean length of dry spells (MDS) and maximum length of dry spells (MxDS), and four periods (spring, summer, autumn and winter), are considered. The results show that a predominant east to west increasing gradient for mean MDS and MxDS has been found in all periods. Mean NDS shows an opposite distribution to that of mean MDS and MxDS. Drought in the western area is more severe than in the eastern area. According to temporal analysis by using the Mann–Kendall trend method, summer is the only season showing strong trends for all the DS indices. During summer, the trends of MDS and MxDS dominantly are positive, while the trends of NDS are dominantly negative. This means that the summer droughts have become increasingly serious in Inner Mongolia.