Interannual Modulation of Diurnal Rainfall over Vietnam by ENSO

El Niño–Southern Oscillation (ENSO) is well recognized for its influence on seasonal rainfall. However, its regionally varying impacts on subdaily convective rainfall, particularly over mountainous regions, are complex and poorly understood. Using ground observations, the latest satellite, and reanalysis data, we examine how concurrent ENSO conditions modulate diurnal rainfall over Vietnam, with a focus on the Central Highlands (CH), which serves as Vietnam’s main coffee-growing region and contributes approximately 25% of the country’s total hydropower potential. We examine the May–September period when diurnal rainfall variability is the largest in the region. Diurnal rainfall originates in the eastern CH around noon and propagates westward, peaking over the region in the afternoon. While the diurnal rainfall phase remains consistent, cold ENSO years exhibit stronger afternoon rainfall and a more pronounced westward propagation than warm ENSO years. These changes are linked to strengthened equatorial easterlies that suppress morning ascent, allowing greater surface heating and stronger day–night thermal contrast. This amplifies vertical motion and low-level moisture flux convergence, reinforcing afternoon rainfall. Enhanced easterlies also facilitate extended westward propagation of diurnal rainfall. These results highlight ENSO’s distinct modulation of diurnal convection over the CH, emphasizing the need to consider regional diversity in diurnal rainfall responses across continental Southeast Asia. SIGNIFICANCE STATEMENT: The Central Highlands (CH) of Vietnam play a key role in the country’s coffee production and hydropower generation. Since rainfall variability strongly affects both sectors, this study examines how the concurrent El Niño–Southern Oscillation (ENSO) influences diurnal rainfall features in the CH. We find that during cold ENSO years, afternoon rainfall is stronger and exhibits more extended westward propagation. These changes occur due to enhanced upward motion and low-level moisture flux convergence, which are driven by anomalous easterly winds associated with the cold ENSO phase. This knowledge can help farmers and energy planners better prepare for short-term weather changes and long-term climate risks.