May/June 2006A Tropical Brew...Hurrican Season Here to Stay?by Marc Rapport
Hot air wafts from the Sahara into the African tropics, storms drift out over the Atlantic and churn west. Waves form, uneven air masses kick off circulation... and it's hurricane season in America.
Hurricanes have been a fact of life since the country's beginnings. Our most costly natural disaster in terms of property and dollar damage? Hurricane Katrina in 2005 at $56 billion and still counting. In lives lost? The Galveston storm of 1900, which killed as many as 8,000 people as it took the city by surprise and drove a miles-long pile of storm-created debris across the south Texas island.
The Gulf of Mexico and the Caribbean Sea spawn as many such storms as the far Atlantic, and together, they make the southern U.S. coastal states some of the most cyclone-prone areas on the globe.
Why is that?
"We're constantly trying to learn more, but we do know that hurricanes start from areas of unsettled weather and thunderstorms in the tropics called disturbances," says Hope Mizzell, state climatologist with the S.C. Department of Natural Resources. "Tropical disturbances most commonly form in one of three different ways. Occasionally, a cluster of thunderstorms will break away from the Intertropical Convergence Zone and become better organized or from a mid-latitude frontal boundary that has made its way into the Gulf of Mexico or off the East Coast. Approximately sixty percent of Atlantic tropical cyclones form from easterly waves moving westward from North Africa. Tropical disturbances reach tropical storm strength at thirty-nine miles an hour of sustained wind, and hurricanes at seventy-four and above."
Of course, much more plays into the formation of cyclonic storms, including water temperatures (80-82 degrees Fahrenheit generally is required to allow formation and intensification to begin), the Gulf Stream and its warm flow through surrounding cooler waters, and steering wind currents aloft.
Powered by the heat of the sea, the fiercest storms wrap themselves around a tightening core, which becomes the eye of the storm. They move across open water, swirling cauldrons of immense power that typically expend themselves only when they hit land, unleashing winds, tornadoes and floods, or fall apart as they move over cold water in the northern Atlantic or central Pacific.
One saving grace: potentially devastating storms are often torn apart by shear, in which upper-level winds remove the tops of thunderclouds. But if they aren't, and the steering currents coming off the East Coast don't push the storm away, as they so often do, the U.S. gets hit somewhere.
"We get hit a lot, actually, and always have," observes Cary Mock, a hurricane geographer at the University of South Carolina. "A lot of people feel global warming has something to do with us getting more and fiercer storms, and I wouldn't deny it could have an impact, but we also have to recognize that there are patterns and cycles to this."
Freddy Vang sees the cycles, too, but the deputy director of the Land, Water and Conservation Division at the DNR is more convinced that global warming is having an impact.
"We do have global change. The air temperature is higher. That means the water temperature is higher. That means more heat is held in the water, and that means there's more energy for the storms to use to start," he says.
"Of course, there also are cycles. Heck, until 1900 or so, they grew oranges on Johns Island. And we may again as the cycles swing back. This affects more than just tropical weather. Besides larger and more storms, we're going to have impacts on agriculture, water supply, you name it."
The 1930s through the 1950s was an active period for hurricanes, Mock notes, while 1960 to 1995 was relatively quiet, although those years did, of course, include Hugo in 1989 and Andrew, until Katrina our nation's worst, in 1992.
The Atlantic basin is now apparently in another upswing - as evidenced by the intensity and duration of the 2004 and 2005 hurricane seasons. Storms have only been given names for about fifty years, and last year, we ran out, getting all the way to Tropical Storm Zeta in the Greek alphabet. However, 2004 was actually one of South Carolina's worst, with seven systems affecting the state and four prompting presidential disaster declarations, according to the state Emergency Management Division.
"But in reality, we haven't been directly hit by a major hurricane since Hugo, so we've been very lucky," Mock says. "Especially since we know from history that these hurricanes can and do occur back to back."
In 2005, for instance, Rita (at its peak, the most intense hurricane ever recorded) followed Katrina by just a few weeks and nearly on the same track. In 1893, a June hurricane in the Beaufort area was followed by the devastating "sea islands storm" in August and then another Category 3 storm a few weeks later in the Charleston area, Mock says, while North Carolina recorded seven hurricanes that year. Much earlier, Charleston was hit by two very strong hurricanes in 1752, including one that may have been a Category 4.
"The city was sparsely populated then," the USC professor says. "Probably just what is now the historic area downtown was built up. Nowadays, even a strong Category Two would flood the downtown if it were a direct hit."
"Hit" is a relative term, Mizzell and other weather experts point out, and so are the oft-cited categories. Called the Saffir-Simpson Scale, hurricanes are ranked from 1 to 5.
"These storms have a wide reach," the state climatologist says. " 'Hit' generally refers to where the eye or center of the storm makes landfall, of course. Hugo made landfall at McClellanville but caused extensive damage up into the Grand Strand as well as all the way through Charlotte.
"And look at Katrina (later classified by NHC as a Category Three). It was a Category Five out in the Gulf but not nearly as strong when it made landfall. That's referring to wind speeds. It still carried a Category Five-strength storm surge right onto the Gulf Coast, and we all saw what happened."
Storms that reach Category 3 (111 mph sustained wind or higher) are considered "major" or "intense." Historically, one out of every three that form hits the United States, Mizzell says, but only three of thirty-two such storms did so between 1995 and 2003.
That changed in the past two years, with three out of six hitting in 2004 and similar numbers in 2005, a pattern expected to continue as the weak El Nino winds from the Pacific continue to provide relatively little shear in the westerly upper-level flows. The primary explanation for the decline in hurricane frequency during El Nino years is wind shear in the environment. In El Nino years, the wind patterns are aligned in such a way that the vertical wind shear is increased over the Caribbean and Atlantic. The increased wind shear helps to prevent tropical storms from developing into hurricanes by essentially cutting off the tops of the storms, ventilating the hot convection towers.
This past year was a record year in many ways, both in terms of numbers and intensity. There were twenty-seven named storms and thirteen hurricanes, seven of them major.
So what's in store for 2006? Forecaster Bill Gray, at Colorado State University, regarded as a pioneer of modern hurricane prognostication, is predicting seventeen named storms and nine hurricanes, five of which will be intense. Average would be ten named storms and six hurricanes, two of them intense.
So it's no time for South Carolinians to stop tracking the tropics. Weather professionals, using dozens of different computer modeling methods, certainly won't and are continuing to work on their accuracy.
Right now, the "1-2-3 rule" is generally accepted, Mizzell says. That means the width of the cone of prediction for the center of a storm is about 100 miles for one day out, 200 miles for two days out and 300 miles for three days out.
"The technology for tracking storms and predicting their forward motion continues to improve," she adds. "In five years, the forecast track error seventy-two hours out may be down to two hundred miles."
One big problem may still remain, Mizzell says. The public still tends to "watch the skinny black line instead of concentrating on that cone of error" in those ever-changing forecast maps, she says.
The effects of landfalling tropical systems are widespread, the state climatologist stresses again, and the storms do veer.
"That's why the forecasts include cones of error," Mizzell says. "It's really hard sometimes to get people to pay attention to that fact of life. Max Mayfield (the current director of the National Hurricane Center in Miami) says he's going to have etched on his tombstone: 'Focus on the cone, not the skinny black line.' "