Storms like Typhoon Vamei aren’t supposed to happen. So when U.S. Navy ships were hit by this tropical cyclone in the South China Sea in December 2001, researchers at the Naval Postgraduate School, Monterey, Calif., decided to take a closer look. “For centuries, sailors haven’t worried about tropical storms near the equator,” says Dr. C. -P. Chang, a meteorology professor at the school, “It’s a rule that cyclones are not supposed to develop there.”
The textbooks say that cyclones such as hurricanes (or typhoons as they’re called in the western Pacific) don’t form within 300 kilometers (about 186 miles) of the equator. Typhoon Vamei proved to be an exception to the rule. It spun up just 150 kilometers (about 93 miles) north of the equator—much closer to Earth’s midriff than any other recorded storm.
NASA researchers found that a combination of topography and meteorology, rather than Earth’s rotation helped jump start the cyclone. A strong blast of air from Asia, called a monsoon surge, funneled rapidly down the South China Sea, reaching the equator in a narrow stretch of ocean between Malay Peninsula and Borneo.
To figure out just what happened and how likely it was to happen again, Chang and two visiting professors from Taiwan analyzed data from several weather models and detailed measurements of wind speed and direction provided by NASA’s QuikSCAT satellite. They discovered that not only did Typhoon Vamei develop in a very unlikely spot, it also got its start in an unusual way.
“In the case of Vamei, everything was just the right size and in the right place and stayed long enough. We calculated that the odds of this happening again are about once every 100 to 400 years,” Chang says.
In the image above QuikSCAT's scatterometer reveals the strong surge of winds coming from the north and the curve they make to help give birth to Typhoon Vamei near the equator (the horizontal blue line) on December 26, 2001. Color indicates wind speed (purple and blue indicate winds, while red a pink indicate high winds—over 60 miles per hour). Arrows show the wind direction.