4. Spatial and Temporal Variation of Climate Extremes in Southwestern China

Analysis of changes in twelve indices of extreme temperature and eleven of extreme precipitation at 110 meteorological stations in southwestern China during 1961–2008 revealed statistically significant increases in the temperature of the warmest and coldest nights, in the frequencies of extreme warm days and nights, and in the growing season length. Decreases of the diurnal temperature range and the number of frost days were statistically significant, but a decreasing trend of ice days was not significant. At a large proportion of the stations, patterns of temperature extremes were consistent with warming since 1961. Warming trends in minimum temperature indices were greater than those relating to maximum temperature. Warming magnitudes were greater on the Xizang Plateau and the Hengduan Mountains than on the Yunnan–Guizhou plateau and in the Sichuan basin, as confirmed by the decrease of the regional trend from west to east. Changes in precipitation extremes were relatively small, and only the regional trends in consecutive wet days, extremely wet day precipitation and maximum 1-day precipitation were significant. These trends are difficult to detect against the larger interannual and decadal-scale variability of precipitation. On the whole, the number of rainy days increased on the Xizang Plateau and in the Hengduan Mountains, but the rainy strength has also increased at lower altitude areas. Analysis of large-scale atmospheric circulation changes reveals that a strengthening anticyclonic circulation, increasing geopotential height, weakening monsoonal flow and vapor transportation over the Eurasian continent have contributed to the changes in climate extremes in southwestern China. The spatial distribution of temporal changes of all climate extreme indices in southwestern China reflects the obvious altitude dependence. Trend magnitudes of temperature extremes are significantly higher for flat stations, and followed by summit, intermountain basin, and valley stations. It is obvious that the larger decreasing trend in summit station, and following is flat stations; whereas the greater increasing trend mainly occurred in valley stations in southwestern China, and the intermountain stations also showed the lower decrease or increase. In addition, the mean contribution of the UHI effect to regional trends of urban stations for cold extremes and warm extremes were 16.0 % and 7.9 %, respectively, based on the preliminary evaluation.