Tropical storms draw heat from gulf to become giants

Hurricane Rita

September 23, 2005|By Frank D. Roylance | Frank D. Roylance,Sun reporter

Hurricanes that blunder into the Gulf of Mexico are a lot like crabs that skitter into a waterman's trap. They'll find nourishment in there, but no safe exit.

"If they get into the gulf, they're going to hit something. There's no way out," said James J. O'Brien, a professor of meteorology and oceanography at Florida State University.

Twice now in 26 days, gulf shore residents have faced ferocious, Category 5 hurricanes, their winds fired up for a time to 175 mph by warm gulf waters.

Katrina slowed to 145 mph - Category 4 on the Saffir-Simpson Scale of Hurricane Intensity - before screeching ashore in Louisiana and Mississippi. It flooded New Orleans, forced its evacuation, and caused widespread, catastrophic damage in both states.

And now Rita. After accelerating rapidly to 175 mph late Wednesday, it was bearing down on the Texas and Louisiana coasts with slower but still-dangerous Category 4 winds. More than a million people have fled inland.

The gulf also nourished Hurricane Camille in 1969, one of only three hurricanes known to have made landfall in the United States with Category 5 winds. The other two struck the Florida Keys in 1935 and South Florida in 1992.

So what is it about the gulf?

Most of the big hurricanes that develop in the Atlantic basin begin their lives as tropical waves and depressions well east of the Caribbean Sea, O'Brien said.

They're spawned between 8 degrees and 20 degrees of latitude north of the Equator, where the sea surface temperatures in summer rise to 86 degrees Fahrenheit or more.

"Hurricanes almost never form if the waters are any cooler than that," O'Brien said.

From the eastern tropical Atlantic, the infant storm systems are blown westward with the same trade winds that carried European explorers across the ocean to the New World. Then, at some point, they veer to the north.

"Their role in the big picture is to take the extra heat the sun has put into the oceans and take it northward so it can radiate out into space," O'Brien said.

Often they will head into the North Atlantic without making landfall, as Hurricane Phillipe did this week, almost unnoticed amid all the worry about Rita.

But this year, a persistent high-pressure system, called the Bermuda High, has blocked storms such as Katrina and Rita from making that northward turn.

"So instead of going into South Carolina, North Carolina, or Baltimore ... they're allowed to just keep going west into the Gulf of Mexico," he said.

After gathering strength in the Bahama Islands, both Katrina and Rita drifted west across southern Florida as Category 1 storms. But when they reached the southeastern gulf, they grew rapidly, stoked by the warm water.

In the summer sunshine, O'Brien explained, the gulf's surface waters heat to as much as 90 or 92 degrees. There are no trade winds there to stir up cooler, deeper water, as there are in the tropical Atlantic and the Caribbean to the south.

The gulf is also warmed by the Loop Current, a deep river of warm water that flows north from the Caribbean. It passes through the straits between Cuba and Mexico, loops clockwise around the gulf, and then exits via the Florida Straits.

All this heat is fuel for the storms. And it's almost inexhaustible.

Unlike the warm surface waters of the Atlantic, the Loop Current's warmth runs as deep as 300 feet in the eastern portion of the gulf, O'Brien said.

That's important because hurricanes stir up the waters they pass over. Where the warm upper layers are shallow, the storms' turbulence will bring deeper, colder water to the surface, cutting off the storms' own energy supply. Satellite images reveal a trail of cool water in a hurricane's immediate wake.

"If a hurricane slows at all, it's going to mix up the water and kill itself," O'Brien said. But where warm waters run deep, as they do over the gulf's Loop Current, hurricanes can keep on growing.

Fortunately for Texans, the layer of warm water is much shallower on the western side of the gulf - perhaps only 50 feet deep, O'Brien said.

That might explain why Rita's top winds began to slow yesterday from 175 mph to 150 mph as the storm moved west toward Texas.

Further weakening is possible, according to the National Hurricane Center. But steering winds in the Gulf of Mexico tend to blow south to north, leaving no way out for the big storm, except by going ashore.

Rita was expected to make landfall tonight or early tomorrow in eastern Texas and southwest Louisiana as a Category 3 storm, with top winds between 111 and 130 mph.

frank.roylance@baltsun.com

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