Typhoon Vamei: An equatorial tropical cyclone formation

[1] Due to the diminishing Coriolis effect, the belt 300 km either side of the equator has been considered tropical cyclone-free. Typhoon Vamei, which developed near Singapore on 27 December 2001, was the first recorded tropical cyclone formation within 1.5 degrees of the equator. The development was the result of two interacting systems, a weak Borneo vortex that drifted into the southern tip of the South China Sea and remained there for four days, and a strong and persistent cold surge that created the large background cyclonic vorticity at the equator. The probability of a similar equatorial development is estimated to be once every 100-400 years.