This study characterizes rainfall and temperature variability for the whole of Vietnam and for climate subregions over 40 years from 1971 to 2010. 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 rate of increase is greater in winter than in summer. Except for the Central Highland, annual average temperatures in southern regions are increasing more rapidly than in the North. The increases in temperatures are statistically significant in most sub-regions; however this is not the case for rainfall. The locations of climate boundaries between sub-regions are also discussed and suggestions for repositioning of these are made. Temperature and rainfall variability are shown to be linked to El Niño-Southern Oscillation on both national and sub-regional scale. This relationship is stronger in lower latitudes and in winter.

Reanalysis and observation data from 1979 to 2010 are used to study the climatological behavior of regional vortices over the Southeast Asian sea–Maritime Continent region (SEAMC). After eliminating tropical cyclones from the International Best Tracks Archive for Climate Stewardship (IBTrACS), significant numbers of vortices remained over the region. The results also show that the vortices, rather than being relatively stationary near the Borneo coast in the winter, were consistently present over the SEAMC throughout the year, migrating from the area of the coast of Vietnam, the Philippines, and the Southeast Asian sea in summer to the island of Borneo region in winter. These vortices can produce significant amounts of rainfall in Vietnam, especially in central coastal regions during the postsummer monsoon (autumn) period.

A new objective approach is proposed to identify the seasonal cycle of rainfall for major monsoon regions of the globe. Mean sea level pressure and low-level zonal wind are combined to establish a standardized monsoon index. The index is able to determine the timing of the summer monsoon rainy season across all major monsoon regions in Asia-Australia, Africa, and the Americas. A detailed application of the index in detecting the monsoon onset-withdrawal phases is demonstrated for the Vietnam-South China Sea region. A significant advantage of the index is that it can be used both for studies of monsoon rainfall variability and for monsoon predictability at seasonal time scales.