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Steer hurricanes? This wheel does

Want to know if a storm will bend our way? Keep an eye on the Bermuda High.

By GRAHAM BRINK
Published June 11, 2006


With hurricane season under way, Floridians want to know whether any storms will slam into the state in coming months.

The science isn’t good enough for precise answers. But keeping an eye on the Bermuda High can help.

The vast high pressure system often plays a starring role during hurricane season by helping to steer hurricanes toward the United States or benignly out to sea.

And it might have an even bigger influence this year.

Researchers have found that the high often settles into position in July and stays put for a few months. Once set, the giant system isn’t quickly moved. Its stability helps forecasters determine generally where hurricanes might go.

It’s an important piece of a large puzzle each season, said Robert Weisberg, a marine sciences professor at the University of South Florida. Its current position east of the U.S. mainland bodes well for Florida this season, he said. But it’s still too early to say for sure.

“If it stays to the east, that’s good,” he said. “The more west it moves, the more nervous we get.”

Atmospheric physicist Steve Smith, who leads catastrophe modeling efforts for Carvill, a reinsurance company, predicted that the Bermuda High will have a greater effect this season than it did in 2005.

Conditions this year could produce more classic Cape Verde storms that form near Africa, he said. Cape Verde storms tend to track more closely to the Bermuda High than storms that form in the southwest Caribbean Sea or the Gulf of Mexico.

“It should have a significant effect this year,” Smith said. “Depending on where it settles, that can be good or not very good at all.”
 Scientific uncertainty

The Bermuda High can stretch more than 2,000 miles across and rise thousands of feet into the atmosphere.

Think of it as an elliptical mass of air that wobbles around the subtropical region of the North Atlantic, changing subtly as it interacts with other weather systems. The system spins clockwise, pushing warm and humid air from the Atlantic and Gulf of Mexico through the eastern United States.

Researchers don’t know for sure what causes the high to settle in different locations or exactly why it’s strong some years and weak in others.

“I wish we knew,” Smith said. “There are many things that we still just don’t understand.”

In the early 1900s, meteorologists recognized that hurricanes in the Atlantic Basin moved north out of the tropics, unless they ran into an area of high pressure, which tended to move storms to the west. They saw that a large high pressure system, often centered over Bermuda, appeared to dominate the western Atlantic in the summer and fall. When the system shifted, the trade winds shifted with it.

Hurricanes can’t go very far on their own, so they hitch rides on those winds. And just like a hitchhiker, a hurricane’s limited transportation options can force it to take a circuitous route.

That’s why hurricanes, no matter how powerful, don’t take a straight course through the Bermuda High’s inner core. Instead, they flow around the periphery, riding the high’s strongest winds. Forecasters sometimes describe the storms as logs floating in a river. They go where powerful currents take them.

“In climatological terms, hurricanes are pretty small events compared to broader scale weather like the Bermuda High,’’ Smith said. “They are like pinballs getting belted around by all the other factors.”

In years when few hurricanes hit the United States, the massive system acts like a goalkeeper deflecting storms away from the coast, Smith said. Despite a significant increase in the average number of storms since the mid 1990s, a below average number struck the United States from 2000 to 2002. One main reason: The Bermuda High steered them away.

In those years, the Bermuda High sets up away from the coast of the United States. The position creates an “alley” between the system’s western edge and the U.S. mainland.

Storms still travel west toward the United States along the Bermuda High’s bottom or southern edge. But eventually the system’s clockwise rotation turns the storms north into the alley.

Instead of striking the United States, they keep turning around the Bermuda High’s western edge and eventually head back toward Europe or break up over cooler northern waters.

“It’s a bit like the storms cling to the (Bermuda High),” said Jennifer Collins, an assistant geography professor at USF. “The sooner the storms can turn north, the less the chance that they will hit the east coast.”

Steering storms to Florida

Florida doesn’t always get so lucky.

The Bermuda High can set up closer to the East Coast, or on top of the eastern United States. In those cases, the “alley” shrinks or disappears and the hurricanes cannot turn north before striking land.

That’s what happened during the 2004 hurricane season, when four hurricanes plowed over Florida. Hurricane Charley, for instance, rode the winds of the Bermuda High into the Gulf of Mexico before it turned east into Punta Gorda.

Last season, the system settled in a similar position but with an extension that sat on top of north and central Florida, protecting the Tampa Bay area.

The system, along with other factors, guided Hurricane Katrina across the Everglades and into the gulf. Katrina rounded the western edge of the system and turned north. A frontal system from the west pinned the hurricane against the Bermuda High’s western edge, helping guide it toward New Orleans.

The system also contributed in another way to last year’s record-setting season.

Forecasters blamed record high sea surface temperatures around many parts of the Atlantic Basin.

Part of the reason for the warmer waters was that the Bermuda High was exceptionally weak in the winter of 2004-05 and did not provide the usual cooling of Atlantic Basin waters. The waters started off above average and nothing kept them from getting even warmer as the summer went along.

The good news is that the Bermuda High was stronger this past winter. The trade winds stirred up the Atlantic and cooled the sea surface. Temperatures remain above average, but not as high as last year at this time.

“That should help keep the early part of the season quieter than last year,” Collins said.

 A slew of factors

The Bermuda High alone cannot be credited for every hit or miss of the United States.

Hurricane paths are influenced by a slew of meteorological conditions. Cold fronts, trade winds, the Gulf Stream, even the storm’s own intensity.

Troughs of flowing air can often shove the Bermuda High or subtly change its shape, like pushing on the side of a giant marshmallow.

In August, for instance, Hurricane Irene looked like it might make landfall in the Carolinas. But another weather system nudged, or weakened, the Bermuda High’s western edge. The change allowed Irene to turn sharply east, away from the coast.

The Bermuda High, once settled, is best used to indicate general areas vulnerable to strikes, Smith said. It helps forecasters determine if the Carolinas are more likely to get hit than, say, South Florida or the Gulf Coast. It doesn’t help much in determining whether Miami is more likely to get hit than Tampa Bay.

“It’s gives a strong indication of where landfalls might occur,” Smith said. “But it’s not that precise.”

Graham Brink can be reached at brink@sptimes.com or (727) 893-8406.

[Last modified June 11, 2006, 01:15:01]


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