Breaking the Typhoon Rules
Strong surge of winds coming from the north and the curve they make to help give birth to Typhoon Varmei near the equator (the horizontal blue line) in December 2001.
April 29, 2003
Storms like Typhoon Varmei 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 Varmei
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.
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
Varmei develop in a very unlikely spot, it also got its start in an unusual way.
Hurricanes typically get their spin from the Coriolis effect,
which is a result of the rotation of Earth. In the Northern
Hemisphere, the Coriolis effect is positive and causes winds to
curve counter- clockwise. In the Southern Hemisphere, it is
negative and makes winds curve clockwise. The Coriolis effect
can turn tropical thunderstorms into twirling hurricanes and
typhoons. At the equator, however, its effect is zero, and it
can't provide the needed spin for cyclones to develop.
In Varmei's case, however, the 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. Due to the combined
effect of the Coriolis effect changing from positive to negative
and the complex terrain, the surge winds typically change
direction from their original southwestward course to bend
toward the east at the equator. In the case of Typhoon Varmei,
a winter monsoon weather system with many thunderstorms happened
to be sitting in the area where the surge winds curved. The
system experienced enough spin to start the cyclone-building
process. It evolved into Typhoon Varmei, a major cyclone with
sustained winds of up to 140 kilometers (about 87 miles) per hour-strong enough
to damage two of the unsuspecting Navy ships that got in its
path.
Rare Tempest in a Teapot
"There is a reason why this kind of storm is very rare," says
Chang. "Everything has to be just right. While the monsoonal
surges, the very strong winds from Asia, are a common winter
occurrence in the South China Sea, they have to encounter the
thunderstorm weather system when it is in the middle of the
water. There is only 500 kilometers (about 311 miles) between the Malay Peninsula
and Borneo - about the size of a small cyclone. Both the strong
monsoon winds and the weather system have to be there together
for a few days. If the storm system drifts over land, the
cyclonic development would stop."
"In the case of Varmei, 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.
Chang and his colleagues were supported by the National Science
Foundation and the Office of Naval Research. They reported their
findings about Varmei in the February 15, 2003, issue of
Geophysical Research Letters.
Contact: JPL/Rosemary Sullivant (818) 393-7490
