The provincial governments of Aceh, North Sumatra (Sumut), and West Sumatra (Sumbar) have officially declared a disaster emergency status. This critical decision was made in response to widespread major floods and devastating landslides that have severely impacted these three provinces.
Beyond localized environmental factors, the primary driver behind the recent extreme weather conditions is Tropical Cyclone Senyar. The Meteorology, Climatology, and Geophysics Agency (BMKG) reported a significant escalation in weather severity around the Malacca Strait. This occurred after tropical cyclone seed 95B intensified, ultimately developing into Tropical Cyclone Senyar, even though the cyclone itself has reportedly shown signs of weakening over the past few days. According to the official BMKG website, this cyclone system is steadily approaching the Aceh mainland at approximately 10 km/h, a trajectory that significantly raises the potential for extreme weather across Aceh and North Sumatra.
What is Cyclone Senyar?
Tropical Cyclone Senyar originated in the warm, conducive waters of the Malacca Strait. At its peak, the cyclone recorded maximum winds of up to 80 km/h and a minimum air pressure of 998 hPa. Its core was precisely observed around 5.0° North latitude and 98.0° East longitude, gradually moving towards the Aceh mainland at a consistent speed of 10 km/h. Teuku Faisal Fathani, Head of BMKG, elaborated that the formation of this cyclone was directly linked to an increased supply of water vapor in the warm Malacca Strait, which subsequently fueled the growth of convective clouds across northern Sumatra.
The immediate impacts of Cyclone Senyar have been severe, triggering very heavy to extreme rainfall in both Aceh and North Sumatra. Meanwhile, Riau and West Sumatra are bracing for potentially moderate to heavy rainfalls. Despite forecasts suggesting a weakening intensity of the cyclone, BMKG has issued a stern warning that the potential for extreme weather remains high throughout the current week.
BMKG has also issued comprehensive warnings to the public:
- Strong winds are anticipated across Aceh, North Sumatra, West Sumatra, the Riau Islands, and Riau.
- Mariners should be vigilant for moderate waves, ranging from 1.25 to 2.5 meters, in the central part of the Malacca Strait, the waters of North Sumatra, and the waters of Rokan Hilir.
- A more severe risk of high waves, reaching 2.5 to 4 meters, is projected for the northern Malacca Strait, Aceh waters, and the Indian Ocean stretching from west of Aceh to Nias.
This impactful cyclone had already begun influencing weather patterns since its initial phase as seed 95B, a critical period during its formation. It has since been under continuous surveillance by the Tropical Cyclone Warning Center (TCWC) Jakarta. BMKG further noted that the intensity of extreme rainfall has been amplified by the simultaneous activity of Equatorial Rossby Waves, which have been active in conjunction with the cyclone’s development. Consequently, strong winds and high waves have become prominent features, with waves between 2.5 to 4 meters observed in the Indian Ocean west of Aceh to the Nias Islands, and moderate waves persisting in the central Malacca Strait.
Beyond Senyar, another atmospheric phenomenon contributing to Indonesia’s complex weather situation is Tropical Cyclone Koto, located in the Philippine Sea. Though not a direct hit, Cyclone Koto exerts an indirect influence, elevating the potential for moderate to heavy rainfall and triggering high waves, potentially reaching up to 4 meters, in the waters north of Kalimantan, Sulawesi, and North Maluku.
BMKG has also highlighted several other active atmospheric phenomena that are collectively intensifying the potential for heavy rainfall across the archipelago:
- A weak La Niña event.
- A negative Dipole Mode.
- The influence of the Madden–Julian Oscillation (MJO).
- Active Kelvin and Equatorial Rossby Waves.
- A general strengthening of the Asian Monsoon.
This confluence of atmospheric conditions significantly increases the vulnerability of Indonesian regions, particularly the western and central parts, to exceptionally heavy rainfall in the upcoming week.
Summary
The provinces of Aceh, North Sumatra, and West Sumatra have declared a disaster emergency following widespread floods and landslides. These severe conditions are primarily driven by Tropical Cyclone Senyar, which originated in the Malacca Strait and is approaching the Aceh mainland. Despite showing signs of weakening, the cyclone has already caused very heavy to extreme rainfall in Aceh and North Sumatra, with the potential for extreme weather remaining high.
Beyond Senyar, other atmospheric phenomena are amplifying extreme weather across Indonesia. Equatorial Rossby Waves have intensified rainfall, and Tropical Cyclone Koto in the Philippine Sea indirectly raises rainfall and wave potential in other regions. Additionally, a weak La Niña, negative Dipole Mode, Madden–Julian Oscillation (MJO), Kelvin Waves, and a strengthened Asian Monsoon collectively increase the vulnerability to heavy rainfall, especially in western and central Indonesia.
