中文摘要：

利用国家气候中心华北地区站点雨季降水数据及美国国家环境预报中心和大气研究中心（NCEP/NCAR）的再分析数据等资料,采用统计诊断方法,分析了1961~2014年华北雨季降水异常偏多（少）年对应的Ni？o3.4指数变化特征,发现华北雨季降水异常偏多年通常发生在El Ni？o结束且当年转为La Ni？a的年份,而华北雨季降水异常偏少年通常发生在赤道中东太平洋冷水位相结束且当年发展成El Ni？o事件的年份。并且在华北雨季降水异常偏多年,7~8月华北平均日降水量超过10 mm的天数约占25%,日降水量在4~10 mm的天数也约占25%,两者所占比率远高于华北雨季降水异常偏少年。对华北雨季降水异常偏多年的同期环流特征分析显示：西北太平洋副热带高压（副高）北跳偏早且有明显阶段性偏北情况,同时东亚高空副热带西风急流北移,从而造成华北雨季天气过程频繁、降水偏多。进一步诊断环流特征与ENSO位相转换的关系发现：相比El Ni？o衰减年,在El Ni？o转为La Ni？a的年份,7~8月500 h Pa日本海到渤海正位势高度距平加强,更符合华北雨季降水偏多年的典型环流特征;同时热带沃克环流显著加强,造成东亚高空副热带西风急流北移更为显著,从而更有利于华北盛夏和雨季降水偏多。文中揭示的现象启示我们在做华北盛夏降水和雨季降水的预测时,需要关注ENSO事件前期的变化速度以及未来的演变特征。

英文摘要：

The changing characteristics of Ni？o3.4 index during 1961–2014 corresponding to much more（less） RSPNC（rainy-season precipitation in North China） were statistically analyzed using RSPNC station data and NCEP/NCAR（National Centers for Environmental Prediction/National Center for Atmospheric Research） reanalyzed data, etc. It was found that most of the years with much more RSPNC were coincident with the ENSO phase-switching years from El Ni？o to La Ni？a, and most of the years with much less RSPNC corresponded to the years when the sea surface temperature anomalies over the central and eastern tropical Pacific changed from negative to positive for El Ni？o phase. In the years with much more RSPNC, days with regional mean rainfall more than 10 mm over North China during July and August accounted for about 25% of the total days, and days with regional mean rainfall between 4–10 mm also accounted for about 25% of the total. Both are far more than that in the years with much less RSPNC. The earlier than normal northward shift of the NWPSH（northwestern Pacific subtropical high）, the evident maintenance of NWPSH at the location more north than its normal position, and the northward shift of the East Asian subtropical jet stream were major reasons for the frequent and heavy rainfall in July and August in the years with much more RSPNC. Compared with the El Ni？o decaying years, the years when ENSO switched from El Ni？o to La Ni？a had stronger positive geopotential height anomaly over 500 h Pa near the Japan Sea during July and August, which was more in accordance with the typical circulation pattern in higher RSPNC years. In addition, compared with the El Ni？o decaying years, the years when ENSO switched from El Ni？o to La Ni？a had stronger Walker circulation during July and August. As a result, the East Asian subtropical jet stream shifted northward more significantly, which was also in favor of more precipitation over North China. The phenomenon inspired us to pay close attention to t