Thursday, October 05, 2006
MIAMI — Hurricane Florence — wide right. Hurricane Gordon — wide right. Hurricane Helene and Tropical Storm Isaac — wide right and wide right.
What is going on here? What is consistently making these storms hook through the distant Atlantic and remain far from land?
And speaking of hooks, are we off the hook for the rest of the hurricane season?
The number of hurricanes expected to occur during a 100-year period based on historical data—light blue area, 20 to 40; dark blue area, 40 to 60; red area, more than 60. Map not to scale. Source: the National Atlas and the USGS
Answer No. 1: A ridge of high pressure over the Atlantic that served as our tormentor in recent years, nudging storms toward us, has become our savior, allowing them to bypass us and move north before they touch land.
Answer No. 2.: The 2006 hurricane season runs until Nov. 30 and we're not in the clear quite yet.
We've been fortunate so far, but nature's Tropical Weather Production Co. usually shifts from the Atlantic to a different assembly line at this stage of the season, and we become more susceptible to storms that develop close to home, especially in the Caribbean.
``At this time of year, Florida is a big target for anything coming from that direction,'' said Chris Landsea, science and operations officer at the National Hurricane Center in West Miami-Dade County.
Exhibit A: Hurricane Wilma. Born in the Caribbean last Oct. 15, it grew into the strongest hurricane on record, then weakened somewhat and crashed through South Florida on Oct. 24.
It was one of eight hurricanes that hit the state in 2004 and 2005.
Though the number of storms this year stacks up as pretty average by historical standards, scientists say the season has been an oddity _ a welcome aberration _ and we remain in the middle of a decades-long period of heightened activity.
And this season clearly is different from the past two. By this time last year, for instance, we were dealing with Rita, the 17th named storm of that season, eight more than have been produced this year.
Scientists cannot fully explain these variations, saying that large-scale forces in the atmosphere and ocean are not yet well understood.
"We know the `what,' but a lot of times we don't know the `why,'" said James Franklin, a forecaster at the hurricane center.
Still, Franklin and others note that a mild El Nino _ warmer than normal sea temperatures in the Pacific _ unexpectedly developed this season and helped create crosswinds that can inhibit storm formation in the Atlantic Basin, which includes the Caribbean and the Gulf of Mexico.
In addition, the atmosphere over much of the Atlantic has been unusually dry and stable. Conversely, the atmosphere over the eastern Pacific has been unusually moist and unstable, giving rise to a large crop of Pacific hurricanes that grow out of the same weather seedlings as Atlantic hurricanes.
``These systems take advantage of whatever fertilizer they encounter,'' said hurricane forecaster Lixion Avila. ``They've found it in the Pacific, but they haven't found much fertilizer in the Atlantic this year.''
Obviously, the fewer storms in the Atlantic, the less chance Floridians have of being steamrolled, but other factors also are at work.
``We've had a nice combination of fewer storms that form farther away and have been steered away,'' Landsea said. Steered away, he said, by atmospheric phenomena so powerful they can toss hurricanes around like rubber balls.
In the absence of these ``steering currents,'' hurricanes drift naturally toward the northwest at 2 to 4 mph.
But that hardly ever happens. More often, tropical systems are pushed along by rivers of air called troughs and nudged here or there by other forces.
One of the main gatekeepers is that ridge of high pressure over the Atlantic that has turned, for now, from foe to friend.
Known to many as the Bermuda High because it hangs out around that island, the area constantly shifts in position around the mid-Atlantic and evolves in shape and strength.
Through much of 2004 and some of 2005, it stretched over the nearby Atlantic and sat close to Florida, strong and sturdy like an atmospheric dam.
Hurricanes Frances and Jeanne, for instance, couldn't push through it, and when they tried to get around it, they stormed into Florida. The reason: Air flows clockwise in high pressure systems, and that tends to pull hurricanes to the left of the high.
This year, however, the Bermuda High has receded toward the east and has developed weaknesses that tropical systems have been able to exploit.
One storm after another — Florence, Gordon, Helene and Isaac — pushed through the Bermuda High or around it, heading more north than west. In other words, away from Florida.
This is lovely for several reasons: Not only did the storms completely avoid land (except for Florence, which clipped Bermuda), but they also turned north so quickly that they didn't have time to drastically intensify over warm southern waters.
So, why have the patterns changed, and how long will they persist?
No one knows and we still have to be alert, especially for something coming at us from the Caribbean.
``But I like seasons like this,'' Landsea said. ``So far.''
(c) 2006, The Miami Herald.
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