The onset and withdrawal dates and the length of Asian summer monsoon period over Indochina from 1979 to 2003 are defined using the transition of water origins between the Indian Ocean and the Pacific Ocean. Mean onset and withdrawal dates for 25 years are 11 May and 17 September, respectively. Mean onset and withdrawal dates are almost same and one month earlier than former definition by using rainfall patterns. Mean length of monsoon period is approximately 130 days. The correlation between onset and withdrawal date is not significant. Also, the correlation between the length of monsoon season and onset date is larger than that for withdrawal date. It suggests that the length of Asian summer monsoon period over Indochina is mainly defined by the onset date. The composite differences in precipitable water evaporated from the Indian Ocean and Pacific Ocean between the early and late phases of the Asian summer monsoon onset and withdrawal were considered. There are north and south anomaly patterns between the Indian Ocean and Pacific Ocean origins in the monsoon onset period. These patterns may suggest the summer and winter monsoon activities affect the large-scale atmospheric circulations.
