INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 34: 3237–3250 (2014) Published online 28 December 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.3908 Climatological onset date of summer monsoon in Vietnam Dzung Nguyen-Le, a * Jun Matsumoto a,b and Thanh Ngo-Duc c a Department of Geography, Tokyo Metropolitan University, Hachioji, Tokyo, Japan b Research Institute for Global Change, JAMSTEC, Yokosuka, Kanagawa, Japan c Department of Meteorology, Hanoi College of Science, Vietnam National University, Hanoi, Vietnam ABSTRACT: 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. KEY WORDS monsoon onset; Foehn wind; summer dry season; Meiyu front; tropical easterly jet Received 2 July 2013; Revised 23 October 2013; Accepted 2 December 2013 1. Introduction Over recent decades, the Asian monsoon has become a critical issue in many studies. Research into the annual cycle of the Asian monsoon system suggests that the seasonal changes in atmospheric circulation are accom- panied by corresponding changes in precipitation (Ram- age, 1971). The Asian summer monsoon (ASM) is often referred to as the rainy phase of a seasonally changing pattern. Therefore, summer monsoon onset is a key indi- cator that characterizes the transition from dry to rainy seasons. For example, Wang and Ho (2002) used rela- tive climatological pentad-mean rainfall to describe the spatial–temporal structure of the Asian–Pacific summer monsoon rainy season. The results showed that monsoon rainfall first increases in the South China Sea (SCS) in mid-May before extending to the northwest Pacific in early to mid-June. Zhang et al. (2002) studied the clima- tology and interannual variations of the summer monsoon onset date (SMOD) over the Indochina Peninsula (ICP) * Correspondence to: D. Nguyen-Le, Department of Geography, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji, Tokyo 192- 0397, Japan. E-mail: nguyen-ledung@ed.tmu.ac.jp by using the observed daily rainfall at 30 stations and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanal- ysis data recorded from 1951 to 1996. The climatolog- ical onset date was determined as May 9, with a stan- dard deviation of 12 days. However, Matsumoto (1997) observed winter maximum precipitation along the central coastal plain of Vietnam, where the distinction between dry and rainy seasons is difficult to determine through rainfall data. Wang and Ho (2002) also suggested that this area exhibits a non-typical pattern of summer monsoon rainfall. However, appropriate determination of the monsoon onset must also depend on wind fields. Orgill (1967) used wind charts recorded during 1936–1964 to define the onset of the ASM over Southeast Asia as the time at which lower tropospheric equatorial westerlies move northward into southern China during the months of May and June. The results showed that the mean onset date in Indochina was May 17 with a range of 33 days. Cheang and Tan (1988) defined the onset of the southwest monsoon over this region as the date on which when both 850 and 700-hPa zonal wind components become positive and remain positive for at least 20 days. Wang  2013 Royal Meteorological Society