Multiple-Scale Interactions during an Extreme Rainfall Event over Southern Vietnam

In August 2019, southern Vietnam suffered its worst flooding to date, which was also associated with recordbreaking extreme rainfall. This study seeks to explain how this extreme rainfall event can be distinguished from normal rainfall events. The bandpass filter applied to the observed rainfall shows that there were significant intensifications of 3–10day variation and 11–60day oscillations during the event. While the latter is characterized by the intensification of westerly flows from the Bay of Bengal to southern Vietnam, the former is related to more complex movements of a series of synopticscale disturbances over the western North Pacific. The notion of multiplescale interactions, inducing the extreme rainfall event is supported by diagnosing the anomalous columnwise moisture divergence over the southern plain region of Vietnam. It is demonstrated that the anomalous convergences associated with the longterm mean moisture and the synopticscale moisture transported by the longterm mean flow are the most important factors for formation of the extreme rainfall event.