Different answers for the N. Atlantic alone and the Northern Hemisphere overallNOAA image. As a result, it left a large wake of cool water behind it, portrayed in the map as an arc of blue.
In a post a few days ago, I wrote about the “wild and wacky Atlantic hurricane season.” And by the measures I described, including five hurricanes in the Atlantic in just three weeks, the sheer size of Hurricane Igor, and several other factors, that characterization was apt.
For additional reasons why the Atlantic hurricane season has been extraordinary, please see Jeff Masters Wunderblog post from Sept. 15. He gives an eye-opening accounting.
But there is another way to look at the hurricane season that I didn’t mention: “accumulated cyclone energy,” or ACE. By that measure too, the hurricane season in the Atlantic has turned out to be pretty wild, thanks to a veritable explosion of activity during the month of September.
It also turns out, however, that when you look at the Northern Hemisphere overall, the 2010 hurricane season has actually been pretty tame — at least as measured by ACE.
The accumulated cyclone energy is an approximation of the amount of energy used in a tropical storm during its entire lifetime. An ACE rating can also be calculated for an entire hurricane season, using the total number of storms in a season, their strengths, and durations.
The ACE rating for an individual storm or for an entire season is in some ways a more illuminating measure than, say, simply the highest speed of a storm, or its lowest barometric pressure. To understand why, consider Hurricane Igor, which lashed Newfoundland on Wednesday.
For a time, Igor was a powerful Category 4 storm, with a central low pressure of just 925 millibars and winds that peaked at 15o miles per hour. But many other hurricanes in history had even lower pressures and stronger winds. Hurricane Wilma, a devastating Category 5 hurricane in 2005 that killed at least 23 people and caused $20 billion in damages in the United States alone, had a low pressure of just 882 mbar — the lowest ever recorded — and peak winds of 185 miles per hour. This makes it one of the most intense hurricanes on record in the Atlantic basin.
But Igor, although not as intense, was notable in its own right: It turned out to be the second largest tropical storm on record in the Atlantic, according to NOAA. (The well referenced Wikipedia entry on Igor ranks it as the largest.) And it also lasted a long time. Igor attained tropical storm status (and gained its name) on September 8th, and with the exception of one day of weakening, maintained its status as a tropical storm or hurricane until September 21.
Combine high winds (if not record setting) with large size and a long duration and you get a whole bunch of storm energy — in other words, a very high ACE number of nearly 43, making it 10th on the ACE list since 1970, according to the the accounting of Ryan N. Maue, a scientist at Florida State University. By comparison, even though Wilma was much more intense and caused much more damage (it had several landfalls during its lifetime), it’s ACE rating was a bit less: 39. (Katrina’s ACE rating was 20.)
According to Maue, so far this year, the total Accumulated Cyclone Energy for the North Atlantic is 78 percent above the 30-year average. (Wild and wacky indeed!) But for the Northern Hemisphere overall, the ACE rating is 34% below average.
What explains this discrepancy?
Sea surface temperatures in the Atlantic have been extremely warm. From Jeff Masters’ blog on August 9:
Sea Surface Temperatures (SSTs) in the Atlantic’s Main Development Region for hurricanes had their warmest July on record, according to an analysis I did of historical SST data from the UK Hadley Center. SST data goes back to 1850, though there is much missing data before 1910 and during WWI and WWII. SSTs in the Main Development Region (10°N to 20°N and 20°W to 80°W) were 1.33°C above average during July, beating the previous record of 1.19°C set in July 2005. July 2010 was the sixth straight record warm month in the tropical Atlantic, and had the third warmest anomaly of any month in history. The five warmest months in history for the tropical Atlantic have all occurred this year.
Meanwhile, the tropical Pacific Ocean has plunged into La Niña conditions at a record pace over the past three months. The result: as of September 3, this La Niña is the second strongest on record for this time of year, according to Klaus Wolter of NOAA’s Earth Systems Research Laboratory. That’s significant because La Niña is characterized by a drop in sea surface temperatures, which is not conducive to the formation of tropical storms.
At the same time, La Niña tends to reduce wind shear in the Atlantic. Wind shear is the enemy of growing hurricanes, so by reducing it, La Niña can contribute to hurricane activity.