Cool Waters, Strong Winds Keep Hurricanes From Hawaii

Richard A. Lovett
for National Geographic News

August 15, 2007

Hurricane Flossie has been downgraded to a tropical storm after brushing past Hawaii's coastline yesterday.

Despite concerns about flooding and wind damage, the storm brought little more than roiling waves and steady rain as its eye moved about 95 miles (153 kilometers) south of South Point on Hawaii's Big Island—the closest to approach the islands in 15 years (Hawaii map).

Winds had dropped to 60 miles (96 kilometers) an hour by 8 a.m. eastern this morning, and the storm is expected to continue weakening.

But the close call was more than just a lucky miss, storm experts say. In general, the Hawaiian Islands are hit by surprisingly few hurricanes.

"We have had three direct or near-direct hits" since 1950, said Raymond Tanabe of NOAA's Central Pacific Hurricane Center in Honolulu. The hurricanes Dot, Iwa, and Iniki hit the island of Kauai in 1959, 1982, and 1992, respectively.

In part, Tanabe said, the islands have been spared because they're a "small target" in the middle of a large ocean.

In addition, the central Pacific is comprised of deep ocean water that tends to be relatively cool by tropical standards.

Because hurricanes feed on energy from warm waters, central Pacific storms are less intense on average compared to those in the Atlantic.

Spinning Top

But other factors are also at work.

Hurricanes are to a large extent pushed eastward by the trade winds. But their counterclockwise rotation produces southward winds on their leading edge.

The interaction of these winds makes the hurricane tend to roll to the right, Tanabe said, like a spinning top brushing against a wall.

The result is that during most of the season, hurricanes head north-northwest.

Normally the trade winds blow steadily from the east, scooting Pacific storms along a track that passes well south of Hawaii.

Flossie's close approach was a rare exception, Tanabe said, as were the three hurricanes that got close to Kauai.

"Iniki and Iwa hit late in the season, when we didn't have as persistent trade winds," he said.

"We had a surface low [a region of low atmospheric pressure] to the west and weak to nonexistent trade winds over the islands, which allowed the hurricanes to curve to the north."

And Iniki and Iwa were especially rare, because normally a strong summer jet stream protects the islands from storms that veer north.

These upper-level jet stream winds tend to rip the tops off the hurricanes and disperse them.

"You're kind of cutting the storm in two," Tanabe said.

Shelter in Place

When hurricanes do strike, Hawaii is vulnerable to catastrophic impacts.

"We're isolated," Tanabe said. "If we require outside assistance, it's going to take a long time to get here, especially if our ports or airports are damaged.

"We can't just get in a car and drive somewhere," he continued. "We have to shelter in place."

Heavy rains can also be a problem.

"Parts of the islands get 400 inches [1,000 centimeters] of rain per year," Tanabe said.

"We can handle quite a bit of rainfall because we're used to it. But we can't handle it as well if it all comes at once."

What's more, when a hurricane like Flossie passes south of the islands, it produces storm conditions on the islands' dry sides.

In addition to dumping heavy rains, such storms batter ocean reefs to the south that are unaccustomed to heavy surf.

"In winter the Hawaiian islands get big surf on [the north] side, so the reefs there are robust," said Randy Kosaki, research coordinator for the Papah'naumoku'kea Marine National Monument in Hawaii's northeastern islands.

(Related: "Hurricanes Heal Reefs in Surprising Cases" [July 2, 2007].)

"The south shores are more sheltered."

The result is that these reefs produce the best coral, but they also take a pounding when the occasional hurricane passes on their side.

"When you get big surf coming from an odd direction," Kosaki said, "you see a lot of broken coral."

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