Hurricanes' Northerly Turn Influenced by Myriad Forces

By Watanabe, June | Honolulu Star - Advertiser, October 24, 2014 | Go to article overview

Hurricanes' Northerly Turn Influenced by Myriad Forces


Watanabe, June, Honolulu Star - Advertiser


Question: Hurricanes that form in the East Pacific travel west for 2,000 miles on a track that would take them south of Hawaii. But when they near Hawaii they tend to turn north or northwest, threatening Hawaii. Why? Why don't they keep traveling west?

Answer: A hurricane's path basically depends on how the wind is blowing.

That, of course, is a simplistic way of explaining the unpredictable paths of these large tropical storms.

For a scientific explanation, we contacted Robert Ballard, science and operations officer for the National Weather Service's Central Pacific Hurricane Center.

He explained that wind flow, for many hundreds of miles, can steer and shape a storm system throughout the entire depth of the system, which can extend from the surface up 40,000 feet or more. That's past the altitude at which most commercial airlines fly, he noted.

"Not only do these winds steer the system, but changes in these winds over time, and over the depth of the atmosphere, can cause very significant changes in the structure of the system," Ballard said. "As the depth of the tropical storm or hurricane changes, so does the layer of winds that steers it."

Meanwhile, the Coriolis force, "an apparent force caused by the earth's rotation," tends to gradually pull tropical cyclones toward the poles over time, he said.

"Finally, changes that occur with the thunderstorms near the center of the system can alter the movement of tropical cyclones as well, particularly over short time ranges."

These "wobbles," known as trochoidal motions, "can be significant -- larger than the size of one of our smaller islands -- and can alter the movement tens of miles over several hours, and are nearly impossible to predict," Ballard said. "Trochoidal wobbles have been known to fool many into thinking the hurricane has made a sudden turn toward a new direction."

Do land masses affect a hurricane's track?

Studies and experience, such as with Hurricane Iselle, "have shown that land masses of certain shapes and sizes can have some minor effects on the movement of tropical cyclones in certain situations, but these effects overall appear to be quite small and do very little to ensure that a particular area is safe from hurricanes," Ballard said. …

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