Nusantara, Indonesia’s new capital city, experienced a rare extreme rainfall event on 27–28 August 2021. This heavy rainfall occurred in August, the driest month of the year based on the monthly climatology data, and caused severe flooding and landslides. To better understand the underlying mechanisms for such extreme precipitation events, we investigated the moisture sources and transport processes using the Lagrangian model HYSPLIT. Our findings revealed that moisture was mostly transported to Nusantara along three major routes: from Borneo Island (BRN, 53.73%), the Banda Sea and its surroundings (BSS, 32.03%), and Sulawesi Island (SUL, 9.05%). Overall, BRN and SUL were the main sources of terrestrial moisture, whereas the BSS was the main oceanic moisture source, having a lower contribution than its terrestrial counterpart. The terrestrial moisture transport from BRN was mainly driven by the large-scale high vortex flow, whereas the moisture transport from the SUL was driven by the circulation induced by boreal summer intraseasonal oscillation (BSISO) and low-frequency variability associated with La Niña. The near-surface oceanic moisture transport from BSS is primarily associated with prevailing winds due to the Australian monsoon system. These insights into moisture sources and pathways can potentially improve the accuracy of predictions of summer precipitation extremes in Indonesia’s new capital city, Nusantara, and benefit natural resource managers in the region.

The impact of the southerly surge’s interaction with the MJO on rainfall in this study was investigated using daily rainfall data from 2140 weather-observation stations. The southern surge, which coincided with the MJO, enhanced rainfall in the western research region, with Yogyakarta seeing the greatest increase at 4.69 mm/day. Meanwhile, the southern surge that occurred without the MJO increased rainfall in the eastern region, with West Nusa Tenggara seeing the greatest rise at 3.09 mm/day. However, the southerly surge has the effect of lowering rainfall in Jakarta, reaching −2.21 mm/day when the MJO is active and −1.58 mm/day when the MJO is inactive. The southerly surge causes extreme rainfall to only occur in a small part of certain areas, so it tends to significantly reduce the possibility of extreme rainfall. In the southern part of the Indonesian maritime continent, the southerly surge predominates over the MJO, supporting increased water vapor transport. Rainfall mostly increases in the afternoon and decreases in the morning when the southerly surge occurs, whether there is the MJO or not. Convective instability analysis indicates that SS increases precipitation, most likely by raising vertically integrated moisture flux convergence, with a correlation coefficient value of 0.82.

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.