Chiang Mai Journal of Science

     Rainfall variability in Thailand is a critical factor for agriculture, water resources, and disaster risk management. The seasonal and regional impacts of the El Niño–Southern Oscillation (ENSO), a significant contributor to interannual variability, remain inadequately characterized at the national scale. This study validates ERA5 monthly precipitation data against observations from Thai Meteorological Department (TMD) stations across four representative regions and examines ENSO-related rainfall anomalies from 1995 to 2024. The validation results indicate that ERA5 effectively reproduces seasonal cycles and interannual variability, with coefficients of determination ranging from 0.80 to 0.87 for annual rainfall across all stations. Overall, ERA5 demonstrates strong performance in tropical regions, underscoring its suitability for climate analysis. The climatological results reveal significant seasonal differences, with the rainy season contributing the majority of annual rainfall. ENSO exerts significant influence on Thai rainfall patterns. El Niño events are associated with reduced precipitation, particularly during summer and monsoon seasons, whereas La Niña events enhance rainfall, especially over the northeastern and central regions. Although winter impacts are less pronounced, they remain evident in the southern peninsula. These findings confirm that ERA5 is a suitable dataset for examining the relationship between ENSO and rainfall in Thailand. They also provide valuable information for early warning systems, agricultural planning, and water resource management.