Typhoon Rare Equator*

Posted 4/5/2003 10:05 AM

Scientists dissect rare typhoon near Equator

WASHINGTON (APOnline) — Weather scientists are dissecting a western Pacific Ocean typhoon that drove home the warning "never say never" by occurring where such storms aren't supposed to happen.

Vamei swirls just east of the Malay Peninsula in late December 2001.

Naval Pacific Meteorology and Oceanography Center

Typhoon Vamei formed in the South China Sea in late December, 2001, about 100 miles north of the Equator.

"The belt 300 kilometers (180 miles) on either side of the equator has been considered tropical cyclone-free," a team of researchers from the Naval Postgraduate School in Monterey, Calif., reports in the online edition of the journal Geophysical Research Letters. This is because the Coriolis effect, by which the earth's rotation gives spin to the wind, is weakest near the equator. In other regions, that spin can more often help form an extreme circular wind storm, known as a typhoon, tropical cyclone or hurricane. (Related: Understanding the Coriolis force)

Vamei "was the first recorded tropical cyclone formation within 1.5 degrees (of latitude, or about 104 miles) of the equator," the researchers said.

Indeed, records going back to 1886 in the Atlantic and 1945 in the Pacific show the closest previous tropical cyclone to the equator was 3.3 degrees for Typhoon Sarah in 1956. One degree of latitude measures about 69 miles, placing Vamei just over 100 miles from the equator.

Typhoon Vamei developed when a weak low pressure area drifted into the South China Sea from the area of Borneo and remained for several days, interacting with a persistent cold surge. The combination generated the turning needed to form the storm, the researchers said.

They estimated the probability of that happening again at once in 100 to 400 years.

Stacy Stewart, a hurricane specialist at the National Hurricane Center in Miami, said that words like "rarely" should be used when speaking about tropical cyclone formation close to the equator, "and never the word 'never'."

The reason for the generality about tropical storms not forming close to the equator is that in the Atlantic the type of low pressure centers that last long enough to turn into a hurricane rarely occur even within 10 degrees of the equator, said Stewart, who previously worked at the U.S. Joint Typhoon Warning Center in the Pacific.

The Monterey team led by C.P. Chang concluded that because cold surges of the type that spurred Vamei reach deepest into the tropics in the South China Sea, "it is unlikely that such a tropical cyclone formation scenario can take place elsewhere along the equator."

But Stewart isn't so sure.

While Vamei was rare, it certainly could happen again, he said

"While the unique geography of Borneo may have helped to spin up the incipient disturbance, I don't feel that this is necessarily unique to only the South China Sea area," he said. "Similar topographical features exist south of the equator throughout Indonesia from Sumatra east to New Guinea, and also in the northern hemisphere west of Colombia."

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