In this study, the climatological summer monsoon onset in Vietnam was investigated by using the 25-year in situ observations and reanalysis gridded dataset recorded from 1979 to 2003. Results suggested that the onset date varies considerably among the sub-regions. Rainfall criteria indicate that the earliest onset generally occurs in the northwestern mountainous area around April 25. That in the northern and southern plains is in mid-May, along with the full establishment of monsoon circulation over the Indochina Peninsula. In the central coastal plain, however, the Foehn wind, an effect of the Asian summer monsoon, causes a dry summer rather than a rainy season. This peculiar characteristic differs substantially from the rainy season in the adjacent regions. Therefore, a new specific criterion for this region is proposed: The summer dry season onset date, which is subjectively indicated in late May, is defined as the appearance time of the dry, downslope Foehn wind.
Monsoon onset in Vietnam and surrounding areas is preceded by the development of convective activity and circulation features over the equatorial Indian Ocean and the southern Bay of Bengal. It also coincides with the northward retreat of mid-latitude westerlies and the eastward retreat of the easterly trade wind associated with the western Pacific sub-tropical high. A relationship between the occurrence of the Meiyu front and the summer monsoon onset over the South China Sea (SCS) is identified. Particularly, cyclonic development concurrent with monsoon onset appears over the SCS and Taiwan area, and a large amount of moisture is observed to converge along the southern side of the front. Simultaneously, the northward extension of the tropical easterly jet (TEJ), which is driven by the reversed poleward temperature and pressure gradient as a consequence of warming over inland India relative to the surrounding ocean, also plays an important role in summer monsoon circulation.