Smartcitizen, at the beginning of this year, Jakarta is entering the peak of the rainy season with high-intensity rainfall. According to the Meteorology, Climatology, and Geophysics Agency (BMKG), this heavy rainfall has been detected since December of last year and is expected to last until February. So, what is causing this? Find the full explanation in this article.

Factors Behind Jakarta’s Rainy Season

The high rainfall in Jakarta does not happen by chance but is influenced by several atmospheric phenomena, such as the Asian monsoon, La Niña, Madden-Julian Oscillation, and Cold Surge.

1. Asian Monsoon

From November 2024 to the present, Indonesia has been undergoing the Asian Monsoon period, which is a seasonal wind blowing from the Asian continent toward tropical regions, including Indonesia. This wind carries significant amounts of moisture from the Pacific and Indian Oceans, thereby increasing the potential for rainfall in various areas, including Jakarta. In January 2025, the Asian monsoon reached its peak in the northern part of Java Island, causing even higher rainfall.

2. La Niña

La Niña is an interaction between the ocean and the atmosphere in the Pacific Ocean that causes sea surface temperatures around Indonesia to be warmer than usual. This condition supports the formation of large rain clouds, increasing rainfall by approximately 20%. Although La Niña this year is considered weaker compared to 2020, there is still the potential for heavy rainfall, which could lead to flooding if not properly managed.

3. Madden-Julian Oscillation (MJO)

The Madden-Julian Oscillation (MJO) is an atmospheric wave that moves across the equator from the Indian Ocean and can influence rainfall patterns in 30 to 60-day cycles. When the MJO is active over Jakarta and the surrounding areas, as is the case now, rainfall increases due to heightened cloud formation activity.

4. Cold Surge

Additionally, there is the phenomenon of cold surge, which refers to the movement of cold air masses from the Asian continent into tropical regions. When a cold surge from the Siberian Plateau meets warm air masses in western Indonesia, it increases the formation of convective clouds, leading to heavy rainfall in various regions, including Jakarta.

5. Equatorial Rossby and Kelvin Waves

Beyond the phenomena mentioned above, there are also influences from Equatorial Rossby and Kelvin Waves, which are atmospheric waves traveling along the equator and affecting cloud formation and rainfall patterns.

• Rossby Waves move from west to east and can slow down the movement of weather systems, causing heavy rain to last longer in a particular region.
• Kelvin Waves, on the other hand, move faster from east to west and can intensify rainfall systems when interacting with other phenomena such as the MJO and cold surges.

Currently, both of these waves are detected as quite active in Indonesia, contributing to increased rainfall in Jakarta and its surrounding areas. A more detailed explanation can also be viewed in an interview with BMKG Head Dwi Korita Karnawati [here].

Stay Alert for Extreme Weather

Although the current La Niña is not as intense as in 2020—meaning the likelihood of extreme rainfall is smaller—according to BMKG Head Dwi Korita Karnawati, the four phenomena mentioned above occur in phases, which could lead to another period of heavy rainfall next month. The impacts could still be significant, especially if preparedness is lacking.

For this reason, the Jakarta Provincial Government and various stakeholders, including Smartcitizens, need to work together to prevent greater consequences.

Smartcitizens are encouraged to remain vigilant and consistently monitor weather forecasts and early warnings through the Pantau Banjir website. The website also provides real-time data on water levels and pump statuses in Jakarta. This information is also accessible via the Pantau Banjir feature in the JAKI app, where it is integrated with other useful features.