Relationship between intraseasonal oscillations and abnormal rainfall in Vietnam

Abstract. Vietnam's summer monsoon season are charaterised by intense rainfall, driven by dynamic intraseasonal oscillations such as the Madden-Julian Oscillation (MJO) and the Boreal Summer Intraseasonal Oscillation (BSISO). Located near the equator with diverse terrain, the country presents a unique case for studying how these atmospheric mechanisms interact with local geography, influencing both the timing and spatial distribution of extreme rainfall events. Despite this, gaps remain in understanding the detailed coupled impact mechanisms that hinder the accurate prediction of abnormal rainfall over the region. This study pioneers an exploration into the interconnected dynamics of abnormal rainfall occurrences and coupled activities of BSISO and MJO in Vietnam. Using association analysis of abnormal events, detected from remote sensing gridded rainfall database, within conditional probability analysis framework, our findings reveal distinct seasonal patterns: during summer, North and South Vietnam experience primary impacts, whereas Central Vietnam becomes more susceptible during autumn. Notably, BSISO phases 3 and 5 dominate the summer landscape, while MJO phases exhibit balanced occurrence frequencies throughout the season. Probability assessments highlight phase 7 of BSISO in July and phases 6–7 in August as periods of heightened extreme rainfall probability in North and South Vietnam, contrasting with phases 5–8 in Central Vietnam. Additionally, MJO phase 5 emerges as a focal point for intensified extreme rainfall in October, alongside notable increases in phases 3–4 during September. This comprehensive analysis enhances our understanding of the complex interactions shaping Vietnam's monsoonal rainfall dynamics, offering valuable insights for future studies levaraging the two intraseasonal osciliations mechanisms to explain and predict abnormal heavey rainfall in Vietnam.