Changes In The Extreme Weather Events And The Responses To The North Atlantic Oscillation Index In The Arid Northwest China

Based on the daily meteorological data in the arid northwest China during 1960-2013, this paper analyzed and studied the spatial distribution and time variation characteristics of extreme climte events in the arid northwest China,Meanwhile, the paper established the Generalized Pareto distribution(GPD) models of extreme precipitation, extreme maximum temperature, and extreme minimum temperature, and introduced the daily North Atlantic Oscillation index(NAOI) as a forcing factor into the models to reveal the influences of North Atlantic Oscillation index on extreme climate events. Primary conclusions are as follows:(1) The spatial distribution of the thresholds of the extreme climte events is uneven in the entire arid northwest China. The threshold of the extreme precipitation is overall small, between 4.7mm and 19.7mm, Large value area are located in the Ili River Valley, North Slope of Tianshan Mountains and southeast of the Hexi Corridor. Small value area are in Jinghe and Turpan region. The extreme maximum temperature threshold is overall higher, between 16.2℃ and 41.7℃, with the large value area located in the Turpan Basin and the surrounding of Tarim Basin and the small value area mainly in bucket turgart and Wushaoling area. The maximum and minimum difference is 25.5 ℃. The extreme minimum temperature threshold value between-10.1℃ and-34.8℃, Small value areas are mainly located in the northern Xinjiang, with high value located in the surrounding of Tarim Basin area. The maximum and minimum difference is-22.5 ℃.(2) The spatial distribution of intensity has a good consistency with the spatial distribution of the threshold of the extreme climate events. The average frequency intensity of extreme precipitation events between 5.0mm and 31.2mm,with the large value area are mainly in the southeastern part of the Hexi Corridor, the north slope Tianshan Mountains and the Ili River Valley areas, and the small value area is in Jinghe and Hongliuhe region, with the maximum and minimum difference 26.2mm. The extreme maximum temperature is high overall, between 17.9℃ and 43.2℃,with the large value area is located in the Turpan Basin and the surrounding Tarim Basin; the small are mainly located in turgart and Wushaoling area. The maximum and minimum difference is 25.3 ℃.The intensity of extreme minimum temperature between-12.5℃ and-38.1℃,with the high intensity areas are mainly distributed in northern Xinjiang, and those in southern Xinjiang and Hexi Corridor are weak, with the maxima and minima difference is-23.5 ℃.(3) The spatial distribution of yearly occurrence frequency shows a significant rising trend in the extreme precipitation and the extreme maximum temperature in most stations of the arid region of Northwest China.The extreme minimum temperature events showed a consistent downward trend significantly.(4) The temporal variations in the exceeds amount and annual frequency of the extreme precipitation and the extreme maximum temperature showed significant rising trends in northern Xinjiang, southern Xinjiang and Hexi Corridor, and those in the extreme minimum temperature events showed a significantly downward trend.(5) The frequency of extreme precipitation existed three types of scales in cycle with 25-30, 10-20 and 5-10 year. The primary cycles in northern Xinjiang, southern Xinjiang and Hexi Corridor were 18 years,29 years and 28 years, respectively. The frequency of extreme maximum temperature presenced three types of scales in cycle, 25-30, 12-20 and 3-10 year, with the primary cycle in northern Xinjiang, southern Xinjiang and Hexi Corridor 29 years, 27 years and 29 years, respectively. The extreme minimum temperature presenced three types of scales in cycle, 20-32, 12-18 and 3-10 year, with the primary cycle in northern Xinjiang, southern Xinjiang and Hexi Corridor 5 years,27 years and 14 years, respectively.(6) The North Atlantic Oscillation Index(NAOI) showed a slight upward trend during 1960-2013. The M-K mutation test found an obvious discontinuities in 1971.There were three types of scales in cycle for NAOI, 20-30, 10-18 and 3-7 year, with the 23-year time scale as the first primary cycle.(7) The responses of frequency and intensity of precipitation to NAOI was significant mainly in the southwest and northeast of northern Xinjiang, the southwest of southern Xinjiang, and the Wuwei region of Hexi Corridor. The respons of high quantiles distribution is not obvious to the NAOI. Both variability and high quantiles distribution has a certain response to NAOI. The site distribution had a good consistency with the response of changes in the frequency and intensity characteristics.(8) The esponses of frequency and intensity of extreme maximum temperature to NAOI was not significant in most areas except Mazongshan region. The respons of high quantiles distribution is not obvious to the NAOI. Both variability and high quantiles distribution has no a certain response to NAOI.(9) The frequency and intensity characteristics of the extreme minimum temperature had an obvious response to NAOI in most parts of northern Xinjiang and few areas of southern Xinjiang and Hexi Corridor. NAOI influenced the high quantiles distribution characteristics weakly, but had an obvious impact on both the variability and the high quantiles distribution characteristics with a good consistency between site distribution and the response of frequency and intensity characteristics to NAOI.