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The calm instead of the storm

November 30, 2006 By Heidi Ledford This article courtesy of Nature News.

Surprise El Nino soothed this year's hurricane season.

2006 was supposed to be a rough year for Atlantic hurricanes. Coming on the back of the devastating 2005 season which saw Hurricane Katrina ravage the US Gulf Coast and forecasters run out of alphabetized names for storms both researchers and the public were braced for an above-average season.

But the Atlantic hurricane season draws to a close on 30 November, a mere five hurricanes after its official start on 1 June. That's three to five short of the number predicted by the US National Oceanic and Atmospheric Administration (NOAA) in May.

"We certainly found this season surprising," says Phil Klotzbach, a research associate at Colorado State University in Fort Collins, whose team also predicted an active season. He has officially labelled this year's predictions a "forecast bust".

There's no indication that this active hurricane era is dissipating.
Gerald Bell, NOAA
Blame a surprise summer El Nio a rise in sea-surface temperatures in the equatorial Pacific Ocean, which can also affect wind patterns in the Atlantic. Such winds can rip apart a fledgling storm and prevent it from intensifying. Summer El Nios are particularly tricky to model, says Gerald Bell, lead hurricane forecaster at the National Hurricane Center in Miami, Florida. "We have a lot of predictive skill with El Nios," he says, "but predicting them during the summer months is still a difficult forecast challenge."

Second guessing

Along with El Nio, Klotzbach also credits an unexplained flow of dry dusty air across the tropical Atlantic in August with keeping the storm-count low. Although the specific reason for the flow is unclear, Klotzbach speculates that it could be due to a particularly dry season in the southern part of the Sahel region in Africa.

Back in May, it looked to be a much fiercer hurricane season. Sea surface temperatures were warmer than average, which could potentially transfer more energy to developing storms. Changes in wind direction, which can nip a hurricane in the bud by fragmenting the storm before it fully develops, were expected to be low. And the Atlantic has seen an above-average hurricane season since 1995.

In May, both NOAA and the Colorado State University team issued higher-than-average forecasts (see box). By August, both teams of researchers had revised their numbers down, but still warned of trouble on the horizon.

Bell says that signs of a small El Nio were already in place at that time. But El Nio activity unexpectedly surged between August and September, catching forecasters off guard and dampening hurricane activity during the usual peak of the season.

Stormy weather

Of the five hurricanes produced in the 2006 season only two developed into 'major' hurricanes Gordon and Helene with winds exceeding 178 kilometres per hour, placing them in category 3 or above. That's the lowest count since 1997, when a powerful El Nio kept all but one category-3 hurricane from forming.

This season, the United States was hit by three tropical storms Alberto, Beryl, and Ernesto and no hurricanes. The bulk of this year's damage came from Ernesto, which caused about $100 million worth of damage to North Carolina and Virginia. That number pales in comparison to last year's Hurricane Katrina, which claimed at least 1,500 lives and caused more than $80 billion in damages.

And the 2005 season was long, too Tropical Storm Zeta, the season's last gasp, didn't die out until 6 January 2006. This year Bell expects no such after-hours activity.

As for 2007, he says it's too early to tell what to expect. "Although we had less activity this year, there's no indication that this active hurricane era is dissipating," he says. "This season should not be interpreted as anything other than a break."

And while a quieter Atlantic hurricane season than last year's (and than expected) may have toned down discussion about global warming intensifying hurricanes, there is no noticeable slackening in tropical storms elsewhere. The eastern Pacific has seen 10 hurricanes this season two higher than the maximum NOAA predicted back in May, and making it an above-average year for that ocean basin.

And as America was putting away its precautions for another year, in Manila they were battening down the hatches for a possible 20cm of rain from supertyphoon Durian, the fourth typhoon to hit the islands in the past three months.

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