The Origins of the Senyar Tropical Cyclone over the Malacca Strait, Indonesia

The rare Senyar tropical cyclone struck Sumatra, Indonesia, on 26 November 2025, resulting in over 1,200 fatalities. This study is mainly interested in developing a vorticity model applied to the low-latitude TC in the Maritime Continent by combining observational, numerical, and theoretical approaches. Senyar originated from an unusual circulation pattern indicated by maximum potential vorticity over the north Malaya Peninsula (6–8 °N), linked to a preconditioned Borneo Vortex and a strong northerly cold surge (CS). The westward deflection of the CS pathway was influenced by elevated sea surface temperature over the Malacca Strait from two days previously. In the same period, the stationary system of vorticity tendency over the Malacca Strait was determined by the rapid horizontal advection from 6°N to the equator. During the precondition phase, an intense and prolonged westerly burst occurred near the equator, followed by a northerly surge. During the peak event, persistent convective clouds up to 5 km developed over the east coast of northern Sumatra, strengthened and connected to rigorous oceanic-based deep convective cloud (~ 20 km) over the Malacca Strait. Besides developing a vorticity model based on a coherent vortex on a β-plane, this study also proposes a simple method to estimate TC impacts, showing that Senyar directly affected about 62,000 km² of Sumatra.