The relationships between the precipitation over East Asia (20°-45°N, 110°-135°E) and the 30-60-day intraseasonal oscillation (ISO) over the Pacific during the boreal summer are studied in the paper. The daily wind and height fields of NCEP/NCAR reanalysis data, the 24-h precipitation data of 687 stations in China during 1958-2000, and the pentad precipitation of CMAP/NOAA from 1979 to 2002 are all analyzed by the space-time filter method. The analysis results, from every drought and flood summer in four different
regions of East Asia respectively during 1958-2000, have shown that the flood (drought) in the East Asian summer monsoon region is absolutely companied with the strongly (weakly) westward propagations of ISO from the central-east Pacific, and depends little on the intensity changes of the East Asian summer monsoon.And the westward ISO is usually the low-frequency cyclones and anticyclones from the Bay of Alaska in northeastern Pacific and the Okhotsk in the northwestern Pacific of mid-high latitudes, and the ISO evolving
in subtropical easterlies. In mid-high latitudes the phenomena are related to the westward propagating mid-ocean trough and the retreat of blocking high. Therefore the westward propagating ISO from the central-east Pacific to East Asia is indispensable for more rainfall occurring in East Asia in summer, which results from the long-wave adjustment process in the mid-high latitudes and ISO evolving in tropical easterlies.