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A NASA computer simulation shows the form that Hurricane Mitch took on Oct. 28, 1998. Mitch, responsible for more than 9,000 deaths, was one of 23 major hurricanes to develop in the Atlantic basin since 1995. Click here or on the image to see how a hurricane is born.
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Since 1995, hurricanes have come rumbling over the Atlantic Ocean more often and with more intensity than during any other time in the last three decades. The catastrophic losses of life and property have prompted many to wonder whether this turn of events reflects a real trend, or just a weird run of bad luck. A study in Friday’s issue of the journal Science concludes that the recent upturn is no coincidence: Atlantic hurricane activity seems to have entered a busy period that could continue anywhere from 10 to 40 years.

Such a shift would have serious consequences. Lulled by the mild hurricane record of the 1970s and 1980s, people have moved to vulnerable coastal areas in droves.

“There’s been so much buildup (along the coast). People have not experienced these storms recently and they’ve forgotten what it’s like,” said the study’s lead author, Stanley Goldenberg of the National Oceanic and Atmospheric Administration’s Hurricane Research Division.

Have humans played a hand?
According to the researchers, the hurricane system swings like a pendulum between more and less active modes of operation, each lasting several decades.

In contrast with previous 24 years, which have been relatively quiet, the last six years have seen a doubling in overall hurricane activity coming from the Atlantic Ocean, with a fivefold increase specifically in the Caribbean.

A commonly asked question is whether this increase might be due to global warming. The new results suggest that the answer is no — at least not primarily. The hurricane track record of the last six years looks similar to the one from the 1920s to the 1960s, when major hurricanes were also relatively common. Goldenberg and his colleagues think the recent upsurge in hurricane activity is more likely the latest swing in the long-term cycle.

“Saying that this increase in activity is from global warming is like saying that it’s getting hot right now because of global warming. Actually, the real reason it’s getting hot is that it’s summer,” Goldenberg said.

The scientists aren’t exactly sure yet what actually drives the cycle that causes hurricane activity to wax and wane periodically. Some researchers have proposed that the global system of ocean circulation may be responsible, but more research is necessary to be certain.

It’s also conceivable that global warming might intensify the current conditions, making the pendulum swing a little higher, so to speak. Goldenberg did notice that this last shift has been more extreme than any other on record. Then again, the difference might also be the result of improved record-keeping.

“These last six years were the most active of all recorded. So, the question is, ‘Gee is that also because it’s also a little warmer than ever before?’ If we’re going to be honest scientists, we just can’t answer that now,” Goldenberg said.

Setting the stage for a hurricane
A hurricane’s center is organized like a chimney, with warm moist air entering at the bottom and then spiraling upward. Two key conditions enable a storm like this to develop.

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First, sea surface temperatures must be relatively warm, in order to feed the hurricane with thermal energy. The second factor, called “vertical wind shear,” is the difference in wind speed and direction between the upper and lower atmosphere. If the vertical shear is too strong, the hurricane can’t assemble itself vertically.

Goldenberg’s research team analyzed historical records of both these conditions. When the scientists looked at how the sea surface temperatures had changed over time, their first result was a collection of haphazard wiggles, seemingly without any logical structure. They used a statistical model to untangle distinct modes, or patterns, in their data, like distinguishing the sounds of individual tones in a piece of music

Once the global effects of the El Niòo cycle — officially known as El Niòo-Southern Oscillation, or “ENSO,” had been removed from the data — the authors could see another regularly occurring cycle in the Atlantic. For several decades, the surface waters would be slightly warmer, meaning conditions would be ripe for hurricane formation; then they’d cool down, switching over to an anti-hurricane phase for several decades more.

“When you go beyond ENSO in the Atlantic, this mode pops out. It’s very strong. These things stick their nose at you,” said Alberto Mestas-Nuòez of the University of Miami, another author of the study.

The timing of the sea surface temperature fluctuations matched similar fluctuations in vertical shear, as well as the actual hurricane record since the 1940s. Thus, Goldenberg’s team may have found the keys behind the long-term waxing and waning of hurricane activity.

More research is still necessary to understand the complex behavior of hurricanes, however. In a commentary article that accompanied the study, Lennart Bengtsson of the Max Planck Institute for Meteorology cautioned against drawing conclusions about long-term trends from the relatively short hurricane record.

Hope for the best, but…
In their study, the Science researchers called on government officials, emergency managers, and coastal residents to prepare for a period in which the hurricane threat is much greater than it was in the 1970s and 1980s.

A particular concern, according to Goldenberg, is that in many coastal areas the buildings have not only become more abundant, they’ve gotten taller as well. Recent observations have shown that hurricane wind speeds can be much higher several stories up than they are at ground level, Goldenberg said. So, the upper stories of a 30-story building could sustain far more damage than the lower floors or a shorter building right next door.

“The saying ‘Hope for the best but prepare for the worst’ really describes the situation,” Goldenberg said. “We can’t say that one particular area is going to be hit by a major hurricane, but these data tell us that there will continue to be more activity and that we certainly need to take the hurricane threat very seriously.”

© 2013 American Association for the Advancement of Science

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