Rainfall in Vietnam
The thesis is a comprehensive analysis of the climate of Vietnam, concentrating on rainfall. Vietnam lies in the tropical northern hemisphere in a region that is influenced by the South Asian, East Asian and Australian monsoons. Rainfall here is associated with several different mechanisms, such as the monsoon, tropical cyclones, topography, and so on. From the initial climatological analysis, monsoon and non-monsoon rainfall is investigated. Non-monsoon rainfall is found to be related to a vortex often found off the coast of Vietnam, a newly-understood extension of the “Borneo vortex”. The thesis begins with a study of the surface climate and the general atmospheric circulation over the Southeast Asian region, which dominates Vietnam’s climate. Trends of surface temperature and rainfall were investigated for a 40 year period (1971-2010), using a newly-extended dataset of 60 stations. Vietnam’s average temperature has increased at a rate of 0.26 ± 0.10°C per decade since the 1970s, approximately twice the rate of global warming over the same period. The increase in temperature is statistically significant in most sub-regions. Trends in rainfall are however mostly insignificant in that period. Temperature and rainfall variability are shown to be linked to ENSO on both a national and sub-regional scale. The rainfall climate of Vietnam has been studied in depth, particularly the characteristics of monsoon rainfall and the variability of the length of wet and dry seasons. That investigation has led to the development of an objective monsoon index, based on regional mean sea level pressure and low level zonal wind. While originally developed to define onset and withdrawal dates for the monsoon in Vietnam and over Southeast Asia, the index is shown to be applicable in all monsoon regions of the globe, the first objective index to have such global utility. The final part of the thesis deals with tropical vortex activity and its associated rainfall. Vortices exist almost year-round, migrating from the coast of Vietnam – Southeast Asian Sea - Philippines in summer to the Borneo Island region in winter. Clear evidence is found for the presence of a semi-permanent vortex near the south coast of Vietnam or the north of the Southeast Asian Sea – Maritime Continent (SEASMC) region. It is this vortex and the tropical cyclones over the SEASMC region that significantly contribute to the Vietnam rainfall, particularly the post-monsoon rainfall in Central Vietnam.