PROBABILISTIC ANALYSIS OF DAILY RAINFALL IN SUMATRA RELATED TO ENSO AND IOD DYNAMICS DURING 1985–2023

Abstract

Sumatra lies between the Pacific and Indian Oceans, making its rainfall variability sensitive to large-scale ocean–atmosphere interactions like the El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD), based on observations from 19 stations of the Indonesian Agency for Meteorology, Climatology, and Geophysics. This study analyzed daily rainfall distribution in Sumatra during 1985–2023 using Probability Density Functions (PDF) for major seasons and ENSO and IOD phase classifications. La Niña and negative IOD enhanced atmospheric convergence and increased water vapor supply in western Indonesia, leading to more moderate to extreme rainfall events. In contrast, a positive El Niño–IOD combination is linked to dry conditions. Monte Carlo testing was conducted with 1000 resampling of neutral years to represent random distribution, testing if rainfall during ENSO/IOD phase differs from neutral conditions. Seasonal samples varied from 8 to 13 years per phase, with significance assessed using 5th and 95th percentiles. PDF analysis indicated a shift toward higher intensity during La Niña and negative IOD, with 18–27% of rainfall intensity bins surpassing the 95% confidence interval. During DJF season, several regions showed high-intensity rainfall frequencies during El Niño that exceeded the confidence interval, highlighting El Niño's role in regional extreme rainfall events. This pattern is explained by the dominance of westerly monsoon and regional convergence, which can modulate the classical ENSO signal. The findings confirmed that Sumatran rainfall response to ENSO–IOD interactions is seasonal, spatial, and nonlinear, with implications for understanding hydrometeorological risks in western Indonesia.