There hasn't been much to talk about, hurricane-wise, in the Atlantic region for quite some time now. All in all, it has been a fairly quiet year, no matter how you stack up the numbers -- and especially if you consider the number and duration of the most intense hurricanes. Several pre-season forecasts were really off the mark on this front, and now, with the season officially over this Friday, it's time to start thinking about why.
The crow-eating began yesterday with a season summary (PDF) from the Colorado State University forecasting team, comprised of Ph.D. student Phil Klotzbach and hurricane guru Bill Gray. They lead with a quotation: "Meteorologists are known to be absolutely brilliant at after-the-fact explanation of weather phenomena ... but please don't press us too hard on future events!" Throughout 2007 Klotzbach and Gray repeatedly forecast an above-average season -- 17 named storms and 5 intense hurricanes, they said in May -- but the year turned out about average: 14 named storms, 2 intense ones.
We shouldn't just beat up on Klotzbach and Gray, though: The National Oceanic and Atmospheric Administration similarly overshot with their 2007 forecasts. Better off were the UK Met Office (they said 7-13 named storms between July and November, and there were 12) and Michael Mann and Thomas Sabbatelli's of Penn State University, who forecast total 15 named storms, plus or minus four in either direction (there were 14).
The relative quietude of the 2007 Atlantic hurricane season seems particularly mystifying in light of the fact that we saw La Nina conditions develop in the tropical Pacific -- which usually correlate with above average Atlantic storm activity. So lets turn to Klotzbach and Gray to hear their reasons for why we had a quiet year. Notably, sea surface temperatures were 0.2 to 0.5 degrees (Celsius) cooler than the average of the past twelve years -- a development that Klotzbach and Gray ascribe to high levels of African dust blown across the ocean, which deflected sunlight. Meanwhile, although La Nina tends to reduce storm-killing vertical wind shear over the Atlantic, this time around it really didn't.
Meanwhile, the month of September was just plain quirky: No less than eight storms formed, but because some were close to land and others just happened to encounter other unfavorable conditions, none developed past short-lived Category 1 hurricane status.
But even on top of all this, October remains the deepest mystery. Or as Klotzbach and Gray put it: "We are still struggling to understand why October was not more active." All the conditions were favorable. But very little happened. It's still not really clear why.
The upshot from all of this: We still have a great deal to learn when it comes to forecasting seasonal hurricane activity in advance. Nevertheless, all forecasters agree that whatever may happen in a given year, we remain in an active hurricane era in general. And even though the United States has now enjoyed a welcome respite for two straight years, there's no reason to expect easy times to continue.
At least until June 2008, however, we can probably rest easy. Enjoy it while it lasts.
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