Abstract: Based on the monthly data of 37 meteorological stations over the Central Yunnan Plateau during 1961-2010, the standardized precipitation evapotranspiration index (SPEI) was used to study the spatiotemporal patterns of droughts. To assess the temporal patterns of droughts as multi-scale events, the SPEI were calculated for seasonal (6 months) and annual droughts (12 months). In this study, the trends of SPEI were detected with linear regression and Mann-Kendall nonparametric test. The morlet wavelet analysis method was used to detect the change interval of annual mean SPEI. The spatial distribution characteristics of SPEI were analyzed via empirical orthogonal function (EOF). The results indicated that: (1) the annual mean SPEI at 37 meteorological stations of the Central Yunnan Plateau was positively correlated with precipitation anomaly percentage and had good sensitivity to the variation of precipitation. Spatial patterns of the SPEI in 2010, 2005 and 1993 over the Central Yunnan Plateau showed good coincidence with the historical drought records; (2) over the Central Yunnan Plateau, annual mean SPEI showed a decreasing trend since the beginning of the 1970s, however an increasing trend was found in the middle of 1990s, and then a continuous decreasing after 2000. The SPEI at 6-month timescale shows a significant decreasing trend in the dry-season (November-April in the following year) in the whole, but there was no significant trend in the rainy season (May-October); (3) the morlet wavelet analysis revealed that the annual mean SPEI fluctuated significantly with the interval of 24 years and 9 years. The Central Yunnan Plateau tends to be dryer after an abrupt changes in annual mean SPEI occur in 1977; (4) from the spatial distribution, four spatial patterns of SPEI variability were objectively defined by the EOF analysis, which were associated with distinct temporal variations and geographical area over the Central Yunnan Plateau. As a result of the influence of large scale climate systems, the annual mean SPEI showed a decreasing trend in the whole research area. Most significant decreasing rate of annual mean SPEI was found in the Southeast parts of the study region. There was no significant trend of annual mean SPEI in the Southwest, whereas the hot-dry valleys along the Jinsha River showed a significant increasing trend of annual mean SPEI during the past five decades.

Key words: droughts, standardized precipitation evapotranspiration index, empirical orthogonal function, Central Yunnan Plateau