Large‐scale patterns and possible mechanisms of 10–20‐day intra‐seasonal oscillation of the observed rainfall in Vietnam

Abstract

The present study investigates 10–20‐day intra‐seasonal oscillation (10–20‐day ISO) of the 29‐year daily observed rainfall in north, central, and south Vietnam in rainy seasons. The 10–20‐day band‐pass Lanczos filter is applied to the time series of the regional daily rainfall, daily reanalysis fields, and outgoing long‐wave radiation. The 10–20‐day ISO in north Vietnam is governed by the alternate occurrence of moisture flux divergence (convergence), which may regulate local lagged conditionally unstable (stable) anomalies. In south Vietnam, the dry (wet) phase is determined by moisture flux divergence (convergence) associated with anticyclonic (cyclonic) gyres that are destroyed later by lagged unstable (stable) anomalies that evolve within the gyres. In both cases, the alternate occurrence of the conditionally unstable and stable anomalies reverses moisture flux and circulations not only over the regions but also in the tropical Indian Ocean in the form of equatorial Rossby waves. The wet phase in central Vietnam is uniquely caused by moisture flux convergence, instead of local unstable condition, which is associated with southwards propagation of cold dry air masses (or cold surges) from the Eurasian continent. Topography also plays an important role in modulating the 10–20‐day ISO, especially in central Vietnam as it helps block northeasterly winds associated with cold air masses.