During the summer of 2017 (June to August),the average precipitation over China was 348.6 mm,which is 8.1%more than normal (332.6 mm).Two rainfall bands were observed over eastern China.The East Asian summer monsoon (EASM) was weaker than normal,while the west Pacific sub-tropical high (WPSH) was significantly stronger than normal with southward ridge position.The "-+-" circulation pattern was located in Eurasian mid-high latitude with positive anomaly over the Lake of Baikal region.The cold SST in the middle-east of equatorial Pacific in the early winter changed to warm phase in the summer of 2017,and the 4th rain pattern,e.g.above normal rainfall in southern China,pre-vailed,while the blocking high over the Lake of Baikal appeared frequently.The major precursory signals of southward rainband are the weakened cold sea surface temperature (SST) in the middle-east of equator Pacific in early winter 2016,the positive triple SST in the North Atlantic in spring 2017,and the decreased snow cover in Eurasia from autumn to winter in 2016.Their common effects benefit the blocking high for-mation over the Lake of Baikal.%2017年夏季(2017年6—8月),全国平均降水量348.6 mm,较常年同期(322.6 mm)偏多8.1%,呈现南、北两条多雨带.东亚夏季风偏弱,西太平洋副热带高压显著偏强,脊线位置偏南;欧亚中高纬呈现"两槽一脊"环流型,贝加尔湖地区为正距平控制.赤道中东太平洋海温从前冬冷水向春、夏季暖水发展,20世纪80年代以来容易出现南方多雨的Ⅳ类雨型,夏季贝加尔湖阻塞高压发展.2017年赤道中东太平洋海温从前冬冷水向春、夏季暖水发展,春季北大西洋三极子正位相和2016年欧亚积雪从秋到冬的减少,均有利于夏季贝加尔湖阻塞高压发展,这是夏季主要多雨带位于长江以南的重要前兆信号.
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