How ENSO controls the propagation speed of seasonal onsets in Java–Bali: satellite rainfall evidence over two decades

Indonesia, as a tropical maritime continent, exhibits a seasonal rainfall pattern influenced by the El Niño–Southern Oscillation (ENSO). Variations in the Wet Season Onset (WSO) and Dry Season Onset (DSO) have broad implications for agriculture, water resources and disaster mitigation. This study aims to analyse the climatology and interannual variability of WSO and DSO over the Java–Bali region (104–116°E), using satellite-based rainfall data (1998–2024). The results show a clear monsoonal pattern, with maximum (minimum) rainfall from December to February (June to August). Climatologically, WSO (DSO) propagates eastward (westward) at ~21 km day–¹ (19 km day–¹) with an onset difference of ~57 (64) days between western Java and Bali. WSO is coherent across all four regions, whereas DSO is coherent across three Java regions (weaker coherence with Bali), consistent with a stronger and more spatially uniform ENSO relationship for WSO. Despite the ENSO predictability barrier, DSO remains significantly related to the Relative Oceanic Niño Index (RONI), with earlier DSO associated more strongly with the developing positive ENSO than with the decaying event from the preceding wet season. El Niño accelerates westward propagation of DSO, whereas La Niña slows DSO and WSO propagation. For WSO, El Niño delays the onset timing with propagation speed comparable with climatology, whereas La Niña advances onset with slower progression. Hence, ENSO affects seasonal onset timing and modulates the longitudinal propagation pace of monsoonal systems. These findings highlight monsoon spatio-temporal propagation as a key atmospheric response to ENSO over Java–Bali, with implications for climate prediction, agricultural planning and hydrometeorological risk management.