Karakteristik Triple-dip La Niña 2020-2023 dan Pengaruhnya terhadap Frekuensi Curah Hujan Lebat hingga Ekstrem di Jawa Barat

Abstract

Triple-dip La Niña 2020–2023 merupakan fenomena ENSO yang penting untuk dikaji karena berlangsung persisten selama beberapa musim, meskipun intensitas puncaknya tidak sekuat Triple-dip La Niña 1998–2001. Persistensi tersebut berpotensi mempertahankan kondisi atmosfer yang lebih basah dari musim ke musim dan meningkatkan peluang terjadinya hujan lebat hingga ekstrem di Jawa Barat. Penelitian ini bertujuan untuk mengidentifikasi karakteristik Triple-dip La Niña 2020–2023, menganalisis perbedaan frekuensi curah hujan lebat hingga ekstrem dibandingkan triple-dip La Niña 1998–2001, La Niña non-triple, dan kondisi netral, serta mengidentifikasi distribusi spasial kejadian hujan lebat hingga ekstrem di Jawa Barat. Data yang digunakan meliputi curah hujan harian dari stasiun BMKG dan pos hujan kerja sama di Jawa Barat, serta data atmosfer–osean berupa Oceanic Niño Index (ONI), suhu permukaan laut (SST), tekanan permukaan laut (SLP), outgoing longwave radiation (OLR), angin zonal 850 hPa (U850), dan angin meridional 850 hPa (V850). Data curah hujan dianalisis untuk periode 1993–2023 setelah melalui tahap pengendalian kualitas. Analisis mencakup klasifikasi periode ENSO, analisis anomali atmosfer–osean, perhitungan proporsi frekuensi curah hujan berdasarkan kategori BMKG, uji chi-square berpasangan dengan koreksi Bonferroni, perhitungan Cramér’s V, serta pemetaan berbasis stasiun. Pendekatan proporsi digunakan agar perbandingan antarperiode ENSO tidak dipengaruhi oleh perbedaan jumlah station-days valid. Hasil penelitian menunjukkan bahwa Triple-dip La Niña 2020–2023 merupakan episode La Niña moderat tetapi persisten. Episode ini berlangsung sekitar 23 bulan dengan nilai ONI minimum sekitar -1,06. Nilai tersebut lebih lemah dibandingkan Triple-dip La Niña 1998–2001, tetapi durasinya lebih panjang daripada sebagian besar episode La Niña non-triple. Selama periode 2020–2023, kondisi atmosfer–osean di sekitar Benua Maritim Indonesia menunjukkan lingkungan yang mendukung kondisi basah, ditandai oleh SST yang relatif lebih hangat, SLP yang lebih rendah, anomali OLR negatif yang menunjukkan peningkatan aktivitas konveksi, serta perubahan angin lapisan bawah yang mendukung transport kelembapan. Analisis frekuensi memperlihatkan bahwa Triple-dip La Niña 2020–2023 memiliki distribusi curah hujan paling basah dibandingkan kelompok ENSO lainnya. Periode ini memiliki proporsi hari kering atau hujan sangat ringan paling rendah serta proporsi hujan ringan dan hujan lebat hingga ekstrem paling tinggi. Proporsi hujan lebat hingga ekstrem mencapai 3,30%, lebih tinggi dibandingkan La Niña non-triple sebesar 3,05%, Triple-dip La Niña 1998–2001 sebesar 2,85%, dan kondisi netral sebesar 2,70%. Uji chi-square menunjukkan bahwa perbedaan proporsi tersebut signifikan secara statistik. Namun, nilai Cramér’s V yang kecil menunjukkan bahwa perbedaan praktis antarperiode ENSO relatif lemah. Oleh karena itu, hasil penelitian diinterpretasikan sebagai peningkatan peluang hujan lebat hingga ekstrem, bukan sebagai lonjakan kejadian yang besar dan merata di seluruh stasiun. Secara spasial, hujan lebat hingga ekstrem pada periode triple-dip La Niña 2020–2023 tidak tersebar merata di Jawa Barat. Hotspot terutama muncul di Jawa Barat bagian selatan dan barat daya, serta sebagian wilayah barat–tengah, antara lain di Cikelat, Cigangsa–Surade, Singajaya, Batu Karut, dan Lanud Atang Sanjaya. Sinyal musiman terkuat terjadi pada SON dan DJF. Pola tersebut menunjukkan bahwa pengaruh triple-dip La Niña berinteraksi dengan faktor lokal, seperti topografi, elevasi, kedekatan dengan Samudra Hindia, sirkulasi monsun, serta transport kelembapan regional. Dengan demikian, dampak triple-dip La Niña 2020–2023 di Jawa Barat lebih tepat dipahami sebagai akumulasi kondisi basah yang persisten, musiman, dan terlokalisasi, bukan hanya sebagai respons terhadap satu puncak La Niña yang kuat.

The 2020–2023 Triple-dip La Niña was an important ENSO phenomenon to examine because it persisted across several seasons, although its peak intensity was not as strong as that of the 1998–2001 Triple-dip La Niña. This persistence may have sustained wetter atmospheric conditions from one season to the next and increased the likelihood of heavy to extreme rainfall in West Java. This study aimed to identify the characteristics of the 2020–2023 Triple-dip La Niña, analyse differences in the frequency of heavy to extreme rainfall relative to the 1998–2001 Triple-dip La Niña, non-triple La Niña, and neutral conditions, and identify the spatial distribution of heavy to extreme rainfall events in West Java. The data used included daily rainfall observations from BMKG stations and partner rain gauges in West Java, as well as atmosphere–ocean data consisting of the Oceanic Niño Index (ONI), sea surface temperature (SST), sea level pressure (SLP), outgoing longwave radiation (OLR), 850 hPa zonal wind (U850), and 850 hPa meridional wind (V850). Rainfall data for the 1993–2023 period were analysed after quality-control procedures. The analysis included ENSO period classification, atmosphere–ocean anomaly analysis, calculation of rainfall-frequency proportions based on BMKG rainfall categories, pairwise chi-square tests with Bonferroni correction, Cramér’s V calculation, and station-based mapping. A proportional approach was applied to ensure that comparisons among ENSO periods were not affected by differences in the number of valid station-days. The results indicate that the 2020–2023 Triple-dip La Niña was a moderate but persistent La Niña episode. It lasted for approximately 23 months, with a minimum ONI value of about -1.06. Although this intensity was weaker than that of the 1998–2001 Triple-dip La Niña, its duration was longer than that of most non-triple La Niña episodes. During 2020–2023, atmosphere–ocean conditions over the Indonesian Maritime Continent were conducive to wetter conditions, as indicated by relatively warmer SSTs, lower SLP, negative OLR anomalies reflecting enhanced convective activity, and lower-tropospheric wind changes that supported moisture transport. Frequency analysis showed that the 2020–2023 Triple-dip La Niña had the wettest rainfall distribution among the ENSO groups. This period had the lowest proportion of dry days or days with very light rainfall, as well as the highest proportions of light rainfall and heavy to extreme rainfall. The proportion of heavy to extreme rainfall reached 3.30%, which was higher than those during non-triple La Niña (3.05%), the 1998–2001 Triple-dip La Niña (2.85%), and neutral conditions (2.70%). Chi-square tests indicated that these proportional differences were statistically significant. However, the small Cramér’s V values suggest that the practical differences among ENSO periods were relatively weak. Therefore, the findings should be interpreted as an increase in the likelihood of heavy to extreme rainfall rather than as a large and uniform increase in rainfall events across all stations. Spatially, heavy to extreme rainfall during the 2020–2023 Triple-dip La Niña was not distributed uniformly across West Java. Hotspots were mainly observed in southern and southwestern West Java, as well as in parts of the western–central region, including Cikelat, Cigangsa–Surade, Singajaya, Batu Karut, and Lanud Atang Sanjaya. The strongest seasonal signals occurred during SON and DJF. This pattern indicates that the influence of the Triple-dip La Niña interacted with local factors, including topography, elevation, proximity to the Indian Ocean, monsoon circulation, and regional moisture transport. Therefore, the impact of the 2020–2023 Triple-dip La Niña in West Java is better understood as an accumulation of persistent, seasonal, and localized wet conditions rather than solely as a response to a single strong La Niña peak.