Hurricanes this past year were unpredictably … average
Lots of experts are weighing in as the Atlantic hurricane season comes to an end (today). One of my favs, Jeff Masters, summarizes it this way:
The Atlantic hurricane season of 2007 is over, and it was a strange one. For the second straight year, we had a near average season, despite pre-season predictions of a very active season.
Before going further, I should point out that hurricane forecasting experts tend to be on the wild side. The dean of forecasters, Bill Gray, has become a cranky global warming denier — you can read his detailed explanation of the 2007 season here (PDF). Masters, on the other hand, flew into hurricanes of his own free will for four years (!), sans parachutes (!!), until he was nearly killed flying into Hurricane Hugo in “the most harrowing flight ever conducted by the NOAA hurricane hunters.”
Now the 2007 season did set a lot of records, as Masters notes:
- Hurricane Felix set the Atlantic record for fastest intensification from the first advisory to a Category 5 hurricane. It took Felix just 54 hours to accomplish the feat.
- Hurricane Humberto set the Atlantic record for fastest intensification from first advisory issued to hurricane strength — 18 hours. (Actually, Humberto did the feat in 14 1/4 hours, but this will get rounded off to 18 hours in the final data base, which stores points every six hours). There have been six storms that accomplished the feat in 24 hours.
- Hurricane Lorenzo tied the Atlantic record for fastest intensification from a tropical depression to a Category 1 hurricane — twelve hours.
- With the occurrence of Dean and Felix, there have now been eight Category 5 storms in the past five years — the highest total ever observed over such a short time span.
- Dean and Felix both made landfall at Category 5 strength, the first time two storms have done that in a single year.
But why were the pre-season forecasts so wrong? Masters explains:
In June, forecasters gave several reasons to expect a very active season in 2007:
- A continuation of conditions since 1995 that have put us in an active hurricane period (in particular, the fact that sea surface temperatures in the Atlantic Main Development Region for hurricanes were about 0.6° C above normal).
- The strong likelihood of either neutral or La Nina conditions in the tropical Pacific Ocean, leading to average to below average wind shear conditions.
Well, La Nina conditions did develop, and wind shear gradually declined during the season. Wind shear was slightly above average in August, near average in September, and below average in October over the main development region for hurricane formation. However, sea surface temperatures declined to near average levels by July and August, thanks to a major incursion of African dust. According to the excellent write-up (PDF) of this hurricane season’s activity posted by Phil Klotzbach and Bill Gray, 2007 was the dustiest year over the tropical Atlantic since 1999. All this dust acted to block sunlight from reaching the ocean surface, and sea surface temperatures were not able to maintain their above average state. We don’t have the ability to predict major dust outbreaks from Africa more than a few days in advance, and this inability will continue to confound efforts at seasonal hurricane prediction for years to come.
The 2007 hurricane season provides no evidence against the theory of human-caused global warming. As we return to normal dust years, expect above-normal hurricane seasons to return.