Makalah ini berisi informasi tentang pentingnya pengelompokkan pola curah hujan yang terjadi di beberapa stasiun penakar curah hujan yang tersebar di P. Sumatera. Hal ini penting dilakukan mengingat kawasan P. Sumatera, umumnya didominasi oleh pola curah hujan Monsunal dengan osilasi dominan sekitar satu tahunan yang dikenal dengan istilah AO (Annual Oscillation). Dengan metode/teknik analisis spektral, maka akan dianalisis apakah benar hampir semua kawasan di Sumatera berpola curah hujan seperti itu. Hasil analisis terhadap tiga puluh tiga stasiun penakar curah hujan yang tersebar di Sumatera Barat selama kurang lebih tujuh tahun pengamatan periode Januari 1986 hingga Desember 1992 menunjukkan bahwa diantara tiga puluh tiga stasiun tersebut, memang benar dua puluh empat diantaranya menunjukkan osilasi satu tahunan (AO). Hal serupa, juga dialami oleh sebagian besar stasiun penakar curah hujan yang tersebar di Sumatera bagian selatan, khususnya kota Palembang. Namun, ada juga beberapa kawasan di Sumetera Barat khususnya yang justru menunjukkan osilasi setengah tahunan yang dikenal dengan istilah SAO (Semi Annual Oscillation). Penjelasan lebih lanjut tentang apa itu osilasi AO dan SAO, termasuk penjelasan tentang metode/teknik spektral yang digunakan, yakni FFT (Fast Fourier Transform) kami bahas secara detail dalam full makalah ini.

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Studi ini menekankan kepada penggunaan teknik FFT (Fast Fourier Transform) dalam menganalisis kenormalan curah hujan bulanan di beberapa kawasan yang ada di Sumatera Barat dan Sumatera Selatan, khususnya pada saat kejadian Dipole Mode (DM). Data yang digunakan selain curah hujan rata-rata bulanan, juga data Dipole Mode Index (DMI) rata-rata bulanan periode Januari 1980 hingga Desember 1999. Hasilnya menunjukkan bahwa secara umum curah hujan bulanan yang tersebar di kawasan tersebut cukup bervariatif. Hal ini ditunjukkan pada analisis spektral yang menunjukkan periodisitas yang cukup kompleks.Hasil analisis spektral lebih lanjut menunjukkan bahwa kedua wilayah tersebut didominasi oleh tipe iklim Monsun. Hal ini terlihat pada puncak energi spektralnya yang berada pada periode sekitar 12 bulan. Walaupun demikian, di beberapa wilayah di Sumatera Barat masih juga ditemukan puncak energi spektral pada periode 6 bulan yaitu di daerah Bukit Tinggi, Maninjau, dan Sicincin. Selain itu, puncak energi spektral juga ditemukan pada periode 18-36 bulan, 50-100 bulan di beberapa wilayah walaupun tidak terlalu kuat.Sementara analisis spektral DMI menunjukkan bahwa puncak energi spektral berada pada periode 18-36 bulan dan 50 bulan. Adanya persamaan osilasi pada DMI dengan curah hujan di kedua kawasan tersebut merupakan indikasi awal bahwa DM

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This study aims to investigate El Niño Modoki phenomenon and its influence to monsoonal rainfall behavior over Indonesia. The study is also intended to identify the differences between the El Niño Modoki and the well-known El Niño events, referred in this study as Conventional El Niño. Power Spectral Density and Wavelet analysis shows a different strength in the temporal cycle of both events, four years interannual cycles of Nino-4 index and nearly decadal (~10 years) cycles of EMI data.

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.

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.

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