INTERAKSI CROSS EQUATORIAL NORTHERLY SURGE (CENS) DAN BORNEO VORTEX (BV) TERHADAP HUJAN LEBAT DI KOTA SEMARANG

Abstract

Hujan lebat pada 30-31 Desember 2022 memicu banjir di Kota Semarang, Jawa Tengah. Penelitian ini bertujuan menganalisis karakteristik hujan serta mekanisme transpor dan konvergensi kelembapan atmosfer yang berkaitan dengan interaksi Cross Equatorial Northerly Surge (CENS) dan Borneo Vortex (BV) pada kejadian tersebut. Data yang digunakan meliputi IMERG GPM, observasi BMKG, dan reanalisis ERA5, kemudian dianalisis melalui model HYSPLIT, Integrated Vapor Transport (IVT), dan Vertically Integrated Moisture Flux Convergence (VIMFC). Hasil menunjukkan hujan sangat lebat hingga ekstrem terjadi pada pagi hari 31 Desember 2022 dengan dua puncak intensitas, yaitu 23 mm/jam pukul 06.00 WIB dan 25,5 mm/jam pukul 09.00 WIB. Pada periode tersebut, CENS aktif dengan angin meridional 925 hPa mencapai -10 m/s, bersamaan dengan berkembangnya sirkulasi siklonik BV di selatan Laut Cina Selatan. Interaksi keduanya memperkuat transpor uap air dari Laut Cina Selatan menuju Laut Jawa dan Semarang. Analisis backward trajectory menunjukkan Laut Cina Selatan sebagai sumber kelembapan dominan dengan kontribusi mencapai 58,63% pada puncak hujan kedua, sementara distribusi VIMFC menunjukkan konvergensi kelembapan yang kuat dan persisten di Semarang selama periode puncak hujan, yang mendukung pembentukan awan konvektif intensif penghasil hujan lebat.

Heavy rainfall on 30-31 December 2022 triggered flooding in Semarang, Central Java, Indonesia. This study aims to analyze the characteristics of rainfall and the mechanisms of moisture transport and convergence associated with this event. The data used include IMERG GPM, BMKG observations, and ERA5 reanalysis data, which were analyzed using the HYSPLIT trajectory model, Integrated Vapor Transport (IVT), and Vertically Integrated Moisture Flux Convergence (VIMFC). The results show that very heavy to extreme rainfall occurred on the morning of 31 December 2022, with two intensity peaks of 23 mm/hour at 06:00 LT and 25.5 mm/hour at 09:00 LT. During this period, an active Cross Equatorial Northerly Surge (CENS), with meridional wind at 925 hPa reaching -10 m/s, coincided with the development of a cyclonic Borneo Vortex (BV) circulation that strengthened meridional moisture transport from the South China Sea, while a zonal flow from the Indian Ocean linked to low-frequency variability also contributed substantially, particularly during the first rainfall peak. Backward trajectory analysis identified the South China Sea as the dominant moisture source during the second peak (58.63%), while VIMFC distribution revealed strong, persistent moisture convergence over Semarang, dominated by the lower atmospheric layer during the peak rainfall period.