Extremes in rainfall and flooding in East Asia occurs mainly during Asian summer monsoon. Like the South Asian monsoon, the East Asian Summer Monsoon system contains low frequency variability with active and break conditions replaced by oscillations of the Mei-yu (in eastern China) and Baiu (in Japan) front (MBF) and by the monsoon trough stretching from northern Indochina to the Philippine Sea. These two major monsoon elements are separated by the North Pacific anticyclone. During the Mei-yu and Baiu time there are two major forms of precipitation in East Asia: stratified and convective. They are both the results of the quasi-stationary front but the latter form appears to contribute more to the extreme rainfall and flooding. The convective elements are partly made of the westerly propagating Rossby waves that emerge from the warm pool of the Philippine Sea. These waves are well depicted in short-range numerical models but little has been done to predict their longer term on 20-25 day time scales. Numerical models have generally failed to forecast the low frequency evolution of the East Asia weather.

Aspects of the structure of the intraseasonal modes of the East Asian summer monsoon are analyzed using satellite rainfall data and reanalysis fields. On a longer term it is found that the Indian and northern Pacific SSTs and ENSO are found to have positive correlation with the Mei-yu-Baiu rainfalls while the Philippine Sea warm pool SSTs have a negative correlation. Their combined influences are studied using wavelet analysis and principal component analysis (PCA). A statistic forecasting model is developed based upon the results of the study. The model will be modified to simulate the intraseasonal variability of East Asian monsoon on the medium range (20-25days) scale.