宁夏夏季极端降水日数的变化规律及其成因

利用1961—2015年宁夏逐日降水资料、NCEP/NCAR再分析资料及NOAA海温资料,分析了宁夏夏季极端降水的变化规律及其成因。结果表明:1)55年来宁夏夏季极端降水日数呈微弱减少趋势,但变率在1994年发生了由大到小的气候突变。各月极端降水的变化存在差异:6月极端降水日数在1982年发生突变,突变后日数显著增多;7月极端降水日数呈微弱减少趋势;8月极端降水日数在1995年发生突变,突变后日数显著减少。2)500 h Pa高度场上6月中国华北—东北与西太平洋上的偶极型异常分布、8月西西伯利亚—蒙古—副热带地区的遥相关波列和EAP异常分布型以及700h Pa上宁夏6月偏东风增强和8月偏北风增强,是导致极端降水事件变化的直接原因。3)6月菲律宾附近海温偏高,有利于500 h Pa高度距平场形成华北—东北与西太平洋的"+-"偶极型异常分布;8月拉尼娜事件的发生,有利于我国呈现西低东高分布型,激发EAP遥相关波列,冷空气与水汽条件相配合,从而导致宁夏极端降水事件频发。

Variation of the days of extreme precipitation in Ningxia in summer and its causes

Based on the daily precipitation data in Ningxia,the NCEP/NCAR reanalysis data and NOAA sea surface temperature(SST) data from 1961 to 2015,this paper analyzes variation of the days of extreme precipitation(DEP) in Ningxia in summer and its causes.Results show that:(1)DEP in Ningxia in summer in the past 55 years has been slightly reduced,but the abrupt change of the variability occurs in 1994 from big to small.Variations of DEP in each month are different.In June,the abrupt change of DEP occurs in 1982,and DEP increases significantly after 1982.DEP in July is slightly decreasing.In August,the abrupt change of DEP occurs in 1995,and DEP decreases significantly after 1995.(2)The dipole pattern in North-Northeast China and the western Pacific at 500 hPa in June,the teleconnection patterns in West Siberia-Mongolia-subtropical China and EAP at 500 hPa in August,as well as wind field at 700 hPa over Ningxia with anomalous easterlies in June and northerlies in August,are the direct reasons leading to the extreme precipitation events.(3)The positive SST anomalies over the Philippines in June are favourable to the "+-" dipole pattern in North-Northeast China and the western Pacific at 500 hPa.The negative SST anomalies over the tropical eastern Pacific in August are good for 500 hPa West-low-East-high height distribution pattern in China,which excites EAP teleconnection wave train.The combination of cold air and water vapor conditions results in the frequent occurrence of extreme precipitation events in Ningxia.