The Gulf Coast’s monster storms - Soundings Online

The Gulf Coast’s monster storms

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Atmospheric and oceanic conditions had to be just right to spawn hurricanes Katrina and Rita

Atmospheric and oceanic conditions had to be just right to spawn hurricanes Katrina and Rita

Twin Category 5 hurricanes don’t spawn less than a month apart and thunder ashore less than 250 miles from each other unless there is just the right witch’s brew of oceanic and atmospheric conditions.

Bathtub-warm water circulating through the Gulf of Mexico for much of August and September helped turn hurricanes Katrina and Rita into the nasty sisters, and a high ridge to the north, over the continental United States, steered the 175-mph storms on nearly identical courses to landfalls on the central Gulf Coast, according to Chris Landsea, science and operations director for the National Hurricane Center.

Rita and Katrina both developed from tropical waves off the lower Bahamas. The clockwise rotation of high pressure to the north sucked the storms northwestward across the Straits of Florida, where they strengthened into Category One hurricanes, says Landsea. Katrina crossed South Florida; Rita swept just south of the Keys. Both popped into the Gulf, where they quickly intensified into Category 5 monsters. The chief culprit responsible for revving up those storms was the Gulf’s loop current.

“You need to have warm water [for a storm to intensify], and not just warm surface water,” Landsea says. To intensify, a storm needs deep reservoirs of warm water. That loop current, a river of tropical Caribbean water, provides that. It pours through the Yucatan Straits, loops clockwise through the Gulf of Mexico, and exits by the Florida Straits to join the northeast-flowing Gulf Stream.

Usually 80-degree water is warm enough to energize a storm. The Gulf’s loop current — a 70- to 80-foot column of water — was 87 degrees F near the surface this summer, and Landsea says that water turbocharged Rita and Katrina. Katrina intensified from a Category 3 to a 5 in just nine hours as it swept north up the Gulf, following that river of warm water to a big warm-water eddy. Rita did the same in less than 24 hours. Satellite photos show the eddy reaching to within 100 miles of the Gulf Coast. Both storms lost strength as they traversed cooler coastal waters before landfall — Katrina winding down to 140 mph and Rita to 120 mph.

“The water [this summer] has been unusually warm,” says Chris Parker, a weatherman for The Caribbean Weather Center, an SSB radio and e-mail forecasting service used by many cruisers. “Sea surface temperatures in the tropical Atlantic, Caribbean and Gulf of Mexico have been 3 to 4 degrees above normal in June, July and August. They’re starting to come back to normal now.”

The elevated water temperature combined with that big eddy — more than 150 miles in diameter — swirling up close to the Louisiana coast likely were factors in spawning these behemoths. Parker cites the high water temperature as a likely factor; Ken Schaudt of Schaudt U.S., another private meteorological service, fingers the eddy.

Landsea points out that historically three-quarters of major hurricanes spawn during the 50-day period from Aug. 20 to Oct. 10. “This time of the year is when you have the best conditions for strong hurricanes to happen,” he says. In addition to warm water, those conditions include:

• Warm, moist air: Warm, moist air rising and cooling and releasing its moisture as rain creates the massive thunderstorms and powerful circulation of air that characterize hurricanes. Masses of dry, cool air from the North damp down a storm, just as water damps down a fire.

• Unstable atmosphere: Instability in the atmosphere results in up- and downdrafts that spawn the thunderstorms and violent circulation of air that can develop into a hurricane.

• Minimum wind shear: A 15-mph easterly at the surface and a 15-mph westerly aloft results in a 30-mph differential — enough “to tear apart a storm,” Landsea says. “It won’t allow the storm to develop.”

• A “trigger”: As a tropical wave moves across the Atlantic from Africa to the Caribbean, it needs a trigger to help it become more organized and develop into a depression. Landsea says the remnants of a cold front may impart just enough spin to begin that process.

“You have to have everything aligned reasonably well,” Schaudt says. “In recent weeks, we’ve seen everything aligned reasonably well.”

The Gulf loop current also has figured into post-hurricane assessments of the environmental damage caused by Katrina and Rita. Mitchell Roffer, a Miami oceanographer who monitors ocean currents and temperatures for anglers, ship captains and oil drilling operators, has been using satellite imagery to track the spread of hurricane runoff through the Gulf, and has posted updates at his company’s Web site . Roffer says this runoff is contaminated with some 8 million gallons of spilled oil, plus pesticides, fertilizers, heavy metals, industrial toxins and sewage. Roffer says a plume of floodwater from the Mississippi River, Mobile Bay and other sources, as well as the surge that receded into the Gulf, has spread across a 200-mile-long by 120-mile-wide swath of coastal waters from Louisiana to Florida, and may be contributing to a resurgence of red tide off Florida. The loop current is carrying a ribbon of this effluent south to the Gulf Stream, reaching the Dry Tortugas by late September. Roffer says this plume off the Keys is 10 miles wide.

“We don’t know how deep it is, but there are parts of it that are 30 miles wide,” he says. “We have seen pools of water from the Mississippi River come down through the Keys, but nothing quite like this.”

He says it is unclear what the environmental impact will be on the Keys’ reefs, fish, turtles and grasses. “We’ve had calls from many people in the fishing community asking, ‘What can we do about it?’ ”

Roffer doubts there is anything that can be done about the spread of the dirty water except track it. “We’re doing daily updates,” he says.