December 1, 2025 | 02:33 pm
TEMPO.CO, Jakarta - Erma Yulihastin, a climate and atmosphere researcher at the National Research and Innovation Agency (BRIN), predicted that the rainy season in Java will be affected by the emergence of the Borneo Vortex in the South China Sea region. The vortex formed from the remnants of Tropical Cyclone Senyar upon its arrival in the waters on Friday, November 28, 2025, after growing in the Malacca Strait and crossing the Malaysian Peninsula.
Erma said the Borneo Vortex would cause relatively dry weather in the southern part of Indonesia. This, she predicted, will last for the first ten days of December. "So, the Java region will tend to have minimal rain because the rain will be concentrated in the South China Sea," Erma told Tempo in an interview on Friday, November 28, 2025.
Erma explained that the Borneo Vortex is a weather phenomenon that occurs almost yearly and affects the Indonesian region. Usually, the atmospheric disturbance in the form of wind swirls that develop near Kalimantan tends to hold back the wet west winds or the Asian Monsoon from reaching Java.
According to the climatology professor, after this period of the Borneo Vortex concludes, likely after the first ten days of December, the rainy season cycle, especially in the Java region, will return to normal. Meaning, Java will continue to have prolonged light to heavy rain that lasts through March 2026.
Recent developments show that the Borneo Vortex has strengthened into a tropical depression, then tropical storm 34 W. Erma urged monitoring its movement and impact on Indonesia's weather and waters over time as a form of mitigation.
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