This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Since 1941 we have never gone later than Sept. 11 without a hurricane forming in the Atlantic Ocean. So when the National Hurricane Center upgraded Tropical Storm Humberto to a hurricane early Wednesday morning, Humberto tied the Sept. 11 record set by Hurricane Gustav in 2002. Gustav later struck North Carolina. Humberto is currently in the tropics between Africa and the Caribbean and is forecast to drift around in the mid-Atlantic, not making landfall.
This year’s record is odd, however, because conditions in the Atlantic have been ripe for storms.
Usually, fewer hurricanes form in the Atlantic when El Niño conditions are present in the Pacific Ocean, because they tend to set up weather patterns that cause shearing winds over the Atlantic that can cut down tropical storms as they grow. Cool Atlantic water also undermines hurricanes. But 2013 is not an El Niño year and water temperatures have been above average, so more storms should have formed by now. On average, three hurricanes coalesce in the Atlantic by Sept. 9. A typical season, which runs from June 1 to November 30, produces six hurricanes, two of which are strong.
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It’s impossible to tell how much water, sun and wind energy exists during the entire six-month season to fuel big storms. Meteorologists use the “accumulated cyclone energy” index as a proxy. ACE is the square of wind speed recorded every six hours in every storm with at least 40 mph sustained winds. Over the past 20 years, ACE has averaged 50 units through Sept. 9, but this year the total was just 9.6 units, according to Jeff Masters, a meteorologist who runs Wunderground.com, a Web site that analyzes and tracks severe weather in great detail. Also strange.
The main reason for the calm, according to Masters, has been a large amount of dry, stable air over the Atlantic, providing less moisture and energy to mounting storms. Much of that air has wafted out over the ocean from the Sahara Desert in Africa. Most Atlantic hurricanes begin as small atmospheric disturbances off the coast of Africa that strengthen as they work their way west and north.
A slow first half of hurricane season is not unusual, however, and it has no bearing on what might happen through the end of November. At least four and as many as nine hurricanes have formed in each of the five years since the 1960s that did not have a hurricane by the end of August. Superstorm Sandy struck the U.S. on Oct. 29 last year. More than 60 percent of Atlantic cyclones form during the second half of the period, so the peak time is just approaching.
As Masters points out on his blog, “the season with the greatest similarity to what we've seen during the first half of the 2013 season was 1988. That year, we also had unusual quietness before September 10, no El Niño, and above average ocean temperatures in the [Atlantic zone where hurricanes form]. But [then] the most powerful Atlantic hurricane ever recorded up to that time ripped through the Caribbean, Hurricane Gilbert, as well as two other major hurricanes.”
And as forecasters at the National Weather Service often say, it only takes one bad storm hitting land to make a bad season.
By the way, in 1941 the date for the first hurricane is Sept. 16, but it may be hard to say how conclusive that record and earlier ones are. The famous Hurricane Hunters began systematically flying over the Atlantic in 1944, and satellite surveillance began in 1960.
Image courtesy of NOAA
