Abstract: The arid region of northwestern China is one of the most sensitive areas to climate change. Climate change have increased the instability of hydrological climate system, resulted in increasing frequency and intensity of climate extremes, as well as shortern the return periods. This paper systematically reviewed the occurrence characteristics, mechanisms and future trends of the extreme climatic/hydrological events in the arid region of northwestern China. Spatial and temporal distribution of precipitation and temperature extremes were analyzed using the Mann-Kendall trend test. The results show that all temperature extremes showed warming patterns, and most of the stations showed significant trends. Warming trends of indices derived from daily minimum temperature were of greater magnitudes than those from maximum temperature, and stations along the Tianshan Mountain had larger trend magnitudes. The number of rainy days had significantly increased. Meanwhile, the precipitation intensity had also significantly increased, which suggested that the increased precipitation in the arid region of northwestern China was due to the increase in both precipitation frequency and intensity. The frequency of extreme hydrological events, such as hail, snowstorm, debris flow, also exhibited an obvious increasing trend, particularly since 1970s. Based on the general circulation models (GCMs), the warm extremes will exhibit increasing trends, while the cold extremes will continue to decrease in intensity, magnitude and duration. However, the simulated results derived from GCMs possess large uncertainties, which lead to larger simulated error in the model-predicted extremes than that in mean variability. The relationships between climate extreme trends and elevation were also discussed, and the suggestion of elevation dependency could be misleading in the arid region of northwestern China. Except for linear trends, another change charactersitic for time series is abrupt change. Results indicate abrupt changes in both cold temperature and precipitation extremes occurred in 1986, while the warm temperature extremes was observed in 1996. The temperature and precipitation extremes had significant change after the abrupt change year. Additionally, teleconnection analysis shows that Northern Hemisphere polar vortex area and Tibetan Plateau High were probably the important factors for changes in both temperature and precipitation extremes in the arid region of northwestern China; extremes in winter were also affected by winter Arctic Oscillation and North Atlantic Oscillation. The general evolution of drought was obtained by rotated empirical orthogonal function, as a result, three drought-related spatial patterns were observed, named North Xinjiang, South Xinjiang, and Hexi Corridor, respectively.The North Xinjiang and South Xinjiang revealed wetter trend, while Hexi Corridor showed a dryer trend. The drying trend over the Hexi Corridor was resulted from a weaker Eastern Asian summer monsoon; whereas, a strong Siberian high-pressure and enhanced airflow from the Baikal Lake resulted in increased precipitation in the Xinjiang regions. This paper is a summary of the current researches, which can provide basis for the in-deep study of climate extremes in the arid region of northwestern China.

Key words: extreme climatic/hydrological events, teleconnections, abrupt change, arid region of northwestern China