Interannual Variation of the Fall Rainfall in Central Vietnam

Abstract

The twenty nine years (1979-2007) rain-gauge based gridded precipitation data generated by the Asian Precipitation−Highly Resolved Observational Data Integration Towards Evaluation of water resources (APHRODITE) are used to depict the rainfall climatology in Vietnam. The rain gauge observations of 163 stations in Vietnam for year 2007 are employed to validate the analysis results of the APHRODITE precipitation and to verify two distinct rainfall regimes: the October-November regime in central Vietnam and the May- October regime in the northern and southern part of this country identified with APHRODITE data. It appears that the Truong Son Range along the western border of Vietnam with Laos and Cambodia provides a natural separation of the October-November rainfall regime in central Vietnam from others. The interannual variation of the October-November rainfall regime can be well depicted by a principal mode obtained from the empirical orthogonal function analysis on the 29-year-APHRODITE precipitation. Time variation of this interannnual mode is out-of-phase with the SST(NINO3.4) index. It is inferred from this negative correlated relationship that central Vietnam is drier (wetter) when the SSTs over the NINO3.4 region is warmer (colder). It is found from the water vapor transport analysis that an anomalous cyclonic (anticyclonic) circulation over south Asia is paired with an anomalous anticylonic (cyclonic) circulation over the western north Pacific during cold (warm) episodes. Water vapor is converged (diverged) by these two anomalous circulations toward (out of ) the South China Sea and Philippine Sea west of 150°E during cold (warm) years. In turn, the anomalous cyclonic (anticyclonic) circulation in South Asia enhances (reduces) the water vapor supply to Indochina, particularly to Vietnam. Coupling with this pair of anomalous circulations, water vapor is converged (diverged) by the anomalous divergent circulation, coupling with the aforementioned pair of anomalous circulation, toward (out of ) Southeast Asia to maintain excessive (deficient) rainfall during cold (warm) episodes. Evidently, the response of the divergent circulation to the tropical Pacific SST anomalies contributes to the interannual variation of the October-November rainfall in central Vietnam.