Flash Floods Hit Bali and East Nusa Tenggara at the Peak of the Dry Season

Flash floods and landslides struck the Mauponggo District, Nagekeo Regency, East Nusa Tenggara (NTT) on Monday (8/9/2025) and Jembrana Regency, Bali on Tuesday (9/9/2025). The floods, triggered by extreme rainfall, occurred at the peak of the dry season, marking the increasing weather volatility due to global warming.

The Regional Disaster Management Agency (BPBD) of Nagekeo Regency is still searching for five missing victims due to flash floods. "The frequently changing weather conditions, coupled with the massive accumulation of materials carried by the flash floods, pose a challenge for the joint Search and Rescue team," said the Head of the Data, Information, and Disaster Communication Center of the National Disaster Management Agency (BNPB), Abdul Muhari.

According to Muhari, the flooding in Nagekeo has caused 18 villages in the Mauponggo District to be isolated due to the disruption of road access, electricity networks, and communication signals. Efforts to distribute logistical assistance will be supported by the NTT Provincial BPBD through sea routes and are scheduled for Thursday (11/9).

Meanwhile, the flooding in Jembrana, Bali has resulted in the deaths of two residents. This disaster occurred following heavy rainfall on Tuesday (9/9) at 11:15 PM WIB. According to data from BNPB, flooding has affected several areas in the Province of Bali. In addition to Jembrana, flooding also occurred in Gianyar, Tabanan, Klungkung, and the City of Denpasar.

"Preliminary data received by BNPB on Wednesday (10/9) at 11:30 AM WIB indicates that two residents have died and 103 families (200 individuals) have been affected in Jembrana Regency. Meanwhile, in Klungkung Regency, a total of 104 families (432 individuals) have been impacted," said Muhari.

Climate researcher from the Meteorology, Climatology, and Geophysics Agency (BMKG) Siswanto stated that August and September should be the peak of the dry season in most of NTT and Bali. "Flooding at the peak of the dry season in the regions of Bali and NTT, as is happening now, has only occurred this time," he said.

According to him, regionally, the heavy rain that triggered the flooding is caused by the active Equatorial Rossby Waves. This has led to fairly widespread heavy rainfall in Bali, Nusa Tenggara, and southwestern Kalimantan.

"Especially in NTT, this is also amplified by local factors in the form of mountainous terrain," said Siswanto.

Rainfall data reported by the NTT Climatology Station indicates that the intensity of rain in Mataloko-Golewa, Ngada Regency, which borders the southern part of Nagekeo, reached 182 millimeters (mm) per day on Monday. Meanwhile, in Ende, the rainfall intensity reached 201.9 mm per day, and in Bajawa, it was 62.1 mm per day.

Regionally, the heavy rain that triggered the flooding was triggered by the activation of the Equatorial Rossby Wave.

A report released by the El Tari Kupang Meteorological Station indicates that sea surface temperatures in the waters of East Nusa Tenggara (NTT) are quite warm and may contribute to cloud formation in the region. At the same time, the equatorial Rossby waves are active, and the easterly monsoon winds are weakening due to the insignificant pressure differences between Asia and Australia. This supports the weakening of winds and enhances the growth of convective clouds in several areas of NTT.

Meanwhile, the rainfall at the Bali Climatology Station on September 9 to 10 reached 385 mm per day, the highest in Indonesia on that day. "This extreme rainfall in Bali is the highest on record in this region," said Siswanto.

According to Siswanto, the phenomenon of Equatorial Rossby Waves is a type of atmospheric disturbance that significantly influences weather patterns in tropical regions, including Indonesia. This wave is named after Carl-Gustaf Rossby, the scientist who first identified it in the 1930s. The main characteristic of Equatorial Rossby Waves is their movement from west to east along the equator at a relatively slow speed, approximately 5–10 meters per second.

This wave typically moves for 10 to 20 days. "This is a planet-scale atmospheric wave, influenced by the rotation of the Earth (Coriolis effect) and the distribution of atmospheric pressure," said Siswanto.

As a result, when active, these waves can cause wind instability in the lower atmosphere and promote the accumulation of moist air masses from tropical ocean areas such as the Indian Ocean and the Java Sea.

Several studies indicate that the impact of equatorial Rossby waves on weather often triggers the growth of intense convective clouds, leading to heavy rainfall and extreme weather, including the recent events in Bali and NTB, which caused flooding and strong winds. The problem is that this phenomenon occurs at the peak of the dry season, which should not support the occurrence of extreme rainfall.

"The problem is that this dry season, the atmosphere and sea in the Indonesian region are warmer than usual, and as a consequence, there is an abundance of water vapor in the atmosphere. This makes it easier to trigger clouds and extreme rainfall. However, certain atmospheric dynamics in the form of weather patterns are still necessary," said Siswanto.

According to Siswanto, the extreme rainfall at the peak of this dry season cannot be separated from the warming of water temperatures due to global warming. "This reinforces the hypothesis in our previous research regarding the increasing risk of extreme weather in Indonesia due to global warming," he stated.

Last week, the World Meteorological Organization (WMO) also reported that the La Niña phenomenon is likely to once again influence weather and climate patterns this year. Although La Niña generally provides a temporary cooling effect and triggers more rainfall in Indonesia, temperatures are still expected to remain above average in most parts of the world.

According to the WMO, there is a 55 percent chance that sea surface temperatures in the equatorial Pacific will cool to La Niña levels. Meanwhile, there is a 45 percent chance that these temperatures will remain at ENSO neutral levels during the period from September to November 2025. For October to December 2025, the probability of La Niña conditions slightly increases to around 60 percent.

Several studies indicate changes in rainfall patterns across various parts of the world due to rising temperatures and humidity. Based on an analysis of satellite data and data from stations worldwide, Lau and Wu (2007), for instance, noted a significant shift in the distribution of rainfall probability functions in tropical regions from 1979 to 2003. There has been an increase in the occurrence of heavy rainfall (the highest 10 percent of precipitation) and light rainfall (the lowest 5 percent of precipitation).

Meanwhile, a study by G Lenderink from the Royal Netherlands Meteorological Institute and his team in Hydrology and Earth System Sciences (2011) found an increase in extreme rainfall over the last century in De Bilt, Netherlands, and Hong Kong. This is related to changes in near-surface humidity. According to the Clausius-Clapeyron theory, every increase in temperature of 1 degree Celsius raises near-surface humidity and extreme rainfall by 7 percent per hour. In Hong Kong and De Bilt, there has been a 14 percent increase.

Siswanto's research, based on long-term data in Jakarta, also found that the increase in temperature has triggered a 14 percent rise in extreme rainfall per hour, which is double the relationship predicted by the Clausius-Clapeyron equation. The temperature in Indonesia has risen by 1.6 degrees Celsius compared to the 1850s.

Therefore, the failure to curb the pace of global warming increases the potential for extreme rainfall in Indonesia. In fact, extreme rainfall can also occur at the peak of the dry season, as seen recently in Bali and NTT, as well as in Jakarta and its surroundings in July and August.