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.

Unusually long duration and heavy rainfall from 5 to 6 February 2021 caused widespread and devastating floods in Semarang, Central Java, Indonesia. The heavy rainfall was produced by two mesoscale convective systems (MCSs). The first MCS developed at 13Z on 5 February 2021 over the southern coast of Sumatra and propagated towards Semarang. The second MCS developed over the north coast of Semarang at 18Z on 5 February 2021 and later led to the first peak of precipitation at 21Z on 5 February 2021. These two MCSs eventually merged into a single MCS, producing the second peak of precipitation at 00Z on 6 February 2021. Analysis of the moisture transport indicates that the strong and persistent north-westerly wind near the surface induced by CENS prior to and during the event created an intensive meridional (southward) tropospheric moisture transport from the South China Sea towards Semarang. In addition, the westerly flow induced by low-frequency variability associated with La Nina and the tropical depression over the North of Australia produced an intensive zonal (eastward) tropospheric moisture transport from the Indian Ocean towards Semarang. The combined effects of the zonal and meridional moisture transport provided favorable conditions for the development of MCSs, and hence extreme rainfall over Semarang. These results provide useful precursors for extreme weather-driven hazard prediction in Semarang and the surrounding regions in the future.

East Nusa Tenggara is one of the provinces in Indonesia that has big forest fires following some provinces in Kalimantan and Sumatra. However, forest fires in East Nusa Tenggara have less attention in forest fires discussion in Indonesia. This study aims to analyze forest fires in East Nusa Tenggara and their impact on reducing visibility and increasing carbon monoxide (CO) from 2015 to 2019. In this study, hotspot, forest fire area, Oceanic Niño Index, visibility, and CO total column data were used to analyze the forest fires using a statistical comparison method in East Nusa Tenggara, Kalimantan, and Sumatra. The result shows that the number of hotspots in East Nusa Tenggara less than in Kalimantan and Sumatra for the same forest fire area. The forest fires in East Nusa Tenggara do not harm the atmospheric environment significantly. East Nusa Tenggara dominantly consists of savanna areas with no peatland, hence, the forest biomass burning produces less smoke and CO. Furthermore, the forest fire in East Nusa Tenggara has not an impact on decreasing visibility and increasing CO total column, in contrast, visibility in Sumatra and Kalimantan has fallen to 6 km from the annual average, and CO total column rise three times of normal condition during peak fire.

Propagasi MJO memicu peningkatan aktifitas konveksi yang menyebabkan kenaikan probabilitas hujan. Intensitas hujan dan frekuensi hujan ekstrem saat MJO pada bulan DJF dan JJA di Indonesia dianalisis. Komposit data curah hujan harian tahun 2008-2018 (CHIRPS) dilakukan berdasar kategori tanggal kejadian MJO (kuat dan lemah) pada tiap fase (3,4,5) menggunakan data indeks Realtime Multivariate MJO. Hujan ekstrem dikategorikan berdasarkan intensitas curah hujan diatas presentil 95%. Hasil menunjukkan MJO kuat Fase 3, 4 dan 5 lebih sering terjadi saat DJF (frekuensi kejadian 50% lebih banyak dibanding saat MJO lemah). Saat JJA, frekuensi kejadian MJO kuat dan lemah tidak berbeda signifikan. Saat DJF, di Indonesia bagian barat terjadi peningkatan intensitas hujan saat MJO kuat Fase 3 dan 4. Di Indonesia bagian timur, peningkatan curah hujan mencapai hampir 100% di beberapa bagian Papua saat MJO kuat Fase 5 DJF. Di sebagian besar Sulawesi saat MJO kuat Fase 4 bulan JJA peningkatan curah hujan mencapai dua kali lipat. Wilayah dengan curah hujan lebih tinggi saat MJO lemah, diantaranya kawasan barat Indonesia (Sumatera dan Jawa) saat MJO Fase 3 di bulan JJA. Hujan ekstrem terjadi baik saat MJO kuat maupun MJO lemah. Frekuensi kejadian hujan ekstrem lebih tinggi saat MJO kuat di Sumatera bagian utara, Jawa bagian timur, Kalimantan bagian selatan, dan beberapa bagian di Pulau Papua saat Fase 3 di bulan DJF, dan pada wilayah Sulawesi dan Maluku saat Fase 4 di bulan JJA. Frekuensi curah hujan ekstrem lebih tinggi saat MJO lemah seperti pada wilayah Papua pada Fase 3 dan 4 bulan JJA.

Nusantara, the Indonesia's New Capital City, experienced a rare extreme rainfall event on 27 to 28 August 2021. This unusual heavy rainfall occurred during the dry season 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, namely 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 acted as the main sources of terrestrial moisture, while the BSS was the main oceanic moisture origin having a lower contribution than that of its terrestrial counterpart. The terrestrial moisture transport from BRN was mainly driven by the large-scale high vortex flow, while the moisture transport from the SUL was driven by the circulations induced by boreal summer intraseasonal oscillation (BSISO) and low-frequency variability associated with La Niña. The near-surface oceanic moisture transport from BSS is mainly associated with prevailing winds due to the Australian Monsoon system. These insights into moisture sources and pathways can potentially improve skill in predictions of summer precipitation extremes in Indonesia's New Capital City, Nusantara, and benefit natural resource managers in the region.