Factors affecting interannual variation in late summer rainfall in the Red River Delta of Vietnam

Abstract Based on station monthly observational statistics, the major rainfall in the Red River Delta (RRD) occurs in late summer (July–September) with conspicuous year-to-year variation. Using the ± 0.8 of the seasonal total rainfall standard deviation as criteria, seven wet and six dry years are identified over the period 1983–2015. In addition to the mean rainfall in wet years being nearly double that in dry years, the occurrence frequency of heavy rainfall days and their corresponding rainfall accumulation in wet (dry) years are higher (lower) than climatology–in particular, the average rainfall accumulation during heavy rainfall days in wet years is more than four times as much as that in dry years. As revealed from further analyses, the large variability in rainfall is attributed to the influence of tropical cyclones (TCs) and 7–24- and 30– 60-day intraseasonal oscillations (ISOs). During the wet (dry) years, the number of TCs affecting the RRD and rainfall produced by TCs are more (less), and the intensity of ISOs also enhances (reduces). It is found from the water vapor budget analyses that an anomalous cyclone (anticyclone) dominates over the Indochina Peninsula in wet (dry) years, resulting in more (less) water vapor being transported to the RRD, whereas the anomalous convergence (divergence) of water vapor flux leads to the maintenance of excessive (insufficient) rainfall over the RRD. However, the El Niño–Southern Oscillation (ENSO) forcing shows minor effects on the interannual variation in rainfall in the RRD.