The northeast Asian summer monsoon (NEASM), which is the subsystem of Asian summer monsoon, covers both subtropics and midlatitudes and its rainfall tends to be concentrated in elongated rain belts (i.e., Mei-yu, Baiu, and Changma frontal bands) that stretch for many thousands of kilometers and affect the regional climate encompassing northeast China, Japan and Korea (20°N-50°N, 100°E-180°E). Undoubtedly, it is crucial to better understand and predict the NEASM rainfall variations to develop the proper management policy of water storage and supply. Connections between the NEASM and tropical Pacific sea surface temperature (SST) have been the long standing subject of many studies (Yang and Lau, 1998; Huang et al., 2004).

A typical El Niņo, which has a peak during the boreal winter (December-January-February, DJF), persistently influences into the next summer (June-July-August, JJA) in circulation and rainfall anomalies in East Asia. It was found that there is a strong biennial signal in the correlations between the East Asian summer monsoon and the tropical Pacific SST (Shen and Lau 1995). In addition to El Niņo, more recent studies have paid attention to the role of Indian Ocean on East Asian monsoon. The anomalous warming or cooling events over the (tropical) Indian Ocean are very important because they influence the climatic variability over the region around the Indian Ocean. Therefore, this study tries to investigate the large-scale characteristics of the strong NEASM due to the Indian Ocean basin mode through the observational composite analysis.