中文摘要：

影响中国、日本、朝鲜半岛的东亚梅雨是夏季风向北推进过程中的特有雨季。利用NCEP／NCAR逐日再分析资料、CMAP降水资料，将夏季风影响及夏季风降水的季节转换相结合，定义东亚梅雨的入、出梅指标；进而采用集合经验模态分解信号提取方法对东亚梅雨区降水季节内振荡及其大尺度环流条件的气候特征进行了详细分析；并对东亚梅雨季节内振荡对降水事件的指示作用进行讨论，为东亚梅雨区降水的延伸预报提供依据和参考。研究结果表明：（1）采用标准化候降水量的空间覆盖率，同时兼顾夏季风影响等条件确定的东亚梅雨入、出梅划分指标可较好地反映东亚梅雨的气候特征及东亚梅雨期的大尺度环流形势。（2）东亚梅雨全年降水量存在三峰型分布特征，峰值分别位于第27、36及47候。该三峰型特征主要受10—20及30—60d的低频振荡影响。比较而言，30-60d振荡对梅雨区降水三峰型的贡献较1020d振荡大。（3）东亚梅雨区峰值降水与热带环流及北方高位涡冷空气输送的低频演变密切关联。在梅雨区北侧，中高纬度里海附近冷空气（高位涡）低频波列的东传及鄂霍次克海高位涡的西南向输送共同影响东亚梅雨区。在梅雨区南侧，通过热带低频异常强对流的激发作用，热带西太平洋至中国东北一鄂霍次克海地区形成沿经向分布的低层气旋－反气旋－气旋－反气旋波列，进而导致梅雨区低层形成低频偏北风和偏南风的辐合；而印度西海岸和阿拉伯海地区异常对流活动产生的波列向东北方向传播，亦对梅雨区低频峰值降水产生影响。对于低频谷值降水的大气低频演变，情况与上述基本相反。（4）东亚梅雨区降水不同位相下出现极端降水事件的概率有明显差异。梅雨区降水低频峰（谷）值位相下出现异常多（少）降水量的概率约为30％。因此

英文摘要：

Meiyu in East Asia including China, Japan and the Korean Peninsula, is the special rainy season during the north- ward movement of East Asian summer monsoon. Using NCEP/NCAR daily reanalysis and CMAP rainfall data, a new index of determining meiyu in East Asia is firstly put forward by combining monsoon influences with rainfall. Then, the Ensemble Empirical Mode Decomposition （EEMD）, the latest signal processing method of nonlinear or non-stationary time series is adopted to indicate intraseasonal oscillation （ISO） characteristics of the climatic variation of rainfall over East Asian meiyu region as well as its relative large scale circulation. And the indicating functions of low frequency oscillation （LFO） of meiyu in East Asia to rainfall events are finally discussed, which may provide basis and reference for extended range forecasts on the intraseasonal time-scale over meiyu region in East Asia. The results suggest that： （1） The new index of East Asian meiyu defined by the spatial coverage percentage with summer monsoon influences involved, can well reflect both the climate features of meiyu rainfall in East Asia and the large-scale circulation conditions during meiyu period. （2） Climatically,the rainfall variation over East Asian meiyu region exhibits a distinct three-peak mode distribution with the peaks at the 27th, 36th and 47th pentad. The three-peak distribution of rainfall amount variation is mainly influenced by 30 - 60 day and 10 - 20 day LFO. Thereinto, 30 - 60 day LFO contributes more to the three-peak mode than 10 - 20 day LFO. （3） Occurrences of peak phases of rainfall ISO over East Asian meiyu region are in close association with both tropical and mid-high latitude LFO. Over the mid-high latitudes, LFO wave train from Caspian Sea and LFO of cold air from Sea of Okhotsk respectively eastward propagate and southwestward transport to the north part of the meiyu region. Over the mid-low latitudes, northward propagation of abnormal convection at low level forms the meridional