Perfect storm. Typhoon Vamei (center) formed so close to the equator in 2001 that its winds swirled in both hemispheres.

The Rarest Typhoon

It's a lesson in Meteorology 101: Hurricanes can't form near the equator. However, a storm called Typhoon Vamei violated that edict in December 2001, arising just 150 kilometers north of the equator in the South China Sea, near Singapore. A new analysis of the strange atmospheric behavior that spawned the typhoon shows that such a storm may occur just once every few centuries.

Hurricanes, called typhoons and cyclones in other parts of the world, are born when intense thunderstorms churn the atmosphere over an expanse of warm ocean water. Earth's rotation makes these disturbances spin by means of the Coriolis effect, an apparent deflection of moving parcels of air that forces storms to whirl counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. This "force" is zero at the equator, so any infant storms there don't get the necessary kick to start spinning. Indeed, no recorded hurricane had formed within about 400 kilometers of the equator.

For that reason, researchers were startled when newborn Typhoon Vamei swept just north of Singapore, at 1.3°N latitude, on 27 December 2001. With sustained winds of 140 kilometers per hour, the cyclone flooded the southern Malay peninsula and damaged two U.S. naval ships. The confluence of events that triggered the typhoon made it close to a perfect storm, say meteorologist C. P. Chang of the Naval Postgraduate School in Monterey, California, and his colleagues in the 1 February Geophysical Research Letters. A vortex of thunderstorms from Borneo drifted over the warm South China Sea and persisted for days. At the same time, an intense surge of cool monsoon winds from the northeast whistled through the gap between Borneo and Indochina, wrapping around the stormy vortex and making it spin with no help from Earth's rotation. The winds and storms lasted just long enough--and the South China Sea was just wide enough--to boost Vamei to typhoon strength. Chang calculates that the unusual conditions might not repeat for 100 to 400 years.

"This was quite an interesting and rare event, both in time and in place," agrees meteorologist Richard Anthes, president of the University Corporation for Atmospheric Research in Boulder, Colorado. Both Chang and Anthes note that cold surges of winds from high latitudes penetrate close to the equator only in the South China Sea, making an equatorial hurricane an unlikely surprise anywhere else on the globe.

Related sites
Abstract of paper by Chang et al.
More images of Typhoon Vamei
Brief report on Vamei from the Malaysian Meteorological Service
Primer on hurricanes and the Coriolis force