The influence of the Madden–Julian oscillation (MJO) on the precipitation extremes in Indonesia during the rainy season (October to April) has been evaluated using the daily station rain gauge data from 1987 to 2017 and the gridded Asian Precipitation–Highly Resolved Observational Data Integration Towards Evaluation of Water Resources (APHRODITE) from 1998 to 2015 for different phases of the MJO. The results show that MJO significantly modulates the frequency of extreme precipitation events in Indonesia, with the magnitude of the impact varying across regions. Specifically, the convectively active (suppressed) MJO increases (decreases) the probability of extreme precipitation events over the western and central parts of Indonesia by up to 70% (40%). In the eastern part of Indonesia, MJO increases (decreases) extreme precipitation probability by up to 50% (40%). We attribute the differences in the probability of extreme precipitation events to the changes in the horizontal moisture flux convergence induced by MJO. The results indicate that the MJO provides the source of predictability of daily extreme precipitation in Indonesia.

Jakarta, the capital megacity of Indonesia located in the northwest of Java Island, Indonesia (Figure 1a), experienced an extraordinary heavy rainfall event in early January 2020. The highest amount reached up to 377 mm (14.83 inches) per day, making it a record-breaking number in observations since 1866. This extreme rainfall subsequently triggered widespread disastrous flooding in Jakarta and its surroundings in the early morning of 1 January 2020, leading to catastrophic losses. It is estimated that at least 173,000 people were evacuated, 66 people died, more than 60% of the residential areas were submerged, and the economic loss reached over US$700 million (Berlinger & Yee, 2020;Nisa, 2020). Because of the high vulnerability of Jakarta to rainfall extremes, a better understanding of the physical processes of heavy rainfall is needed to establish a reliable extended-range flood forecast system for this region.Numerous studies have examined the effects of large-scale atmospheric circulation on precipitation extremes and major floods in Jakarta. For example, the major flooding event in February 2007 (the second highest record-breaking precipitation event) was attributed to an intense and persistent cross-equatorial northerly surge (CENS) that created an intensive low-level wind convergence and favorable dynamic conditions for the

Indian Ocean Dipole (IOD) is a mode of natural climate variability that arises from a zonal-dipole structure in interannual variations of the tropical Indian Ocean. Understanding the influence of IOD on atmospheric circulation is important in order to improve weather forecasts and climate predictions in the Tropics and the extratropics on time scales beyond a few days. This study investigates the role of IOD on convectively coupled equatorial wave (CCEW) activity, including Kelvin and MRG waves and examines the potential links between them. The results show that positive IOD event leads to enhance Kelvin wave and MRG wave activities over the eastern Africa and southern Indian Ocean. On the other hand, it suppresses Kelvin and MRG wave activities over the eastern Indian Ocean and the Maritime continent. It is also shown that IOD’s influence on Kelvin and MRG waves activity is non-dependent to ENSO activity. These results highlight the importance of IOD in generating Kelvin and MRG waves in the tropics.

The relationship between boreal summer intraseasonal oscillation (BSISO) and precipitation extremes over Indonesia is investigated using observational datasets from 30 years of rain gauge measurements and the gridded Asian Precipitation-Highly Resolved Observational Data Integration Towards Evaluation of Water Resources from 1998 to 2015. The results indicate that the frequency of daily extreme precipitation events in Indonesia (defined as total precipitation above the 95th percentile) during extended boreal summer (May-August) is significantly modulated by BSISO, especially over the western and northern regions. Under the influences of BSISO1, the probability of the precipitation extremes over Sumatra and Borneo increases by 20-120% (relative to the seasonal probability) during phases 1-3 and approximately 50-80% over the eastern part of Borneo and Sulawesi during phase 4. Under BSISO2, the probability of the extremes increases up to 40% over Sumatra during phases 1 and 2 and up to 140% over Borneo and Sulawesi during phases 2 and 3. The increase in the probability of extreme summer precipitation is associated with enhanced large-scale moisture flux convergence and upward moisture transport induced by the active phases of BSISO. These results provide potential information for medium-to extended-range predictions of summer precipitation extremes in Indonesia.