As the curtain rises on this year's Atlantic hurricane season, researchers are uncovering important clues as to why storms can sometimes change strength quickly and radically.
In the process, they are working to give forecasters a powerful new tool to help gauge the potential for such rapid changes. The hope is to give people more accurate warnings of a hurricane's "landfall punch" well in advance of its arrival.
While scientists do well in tracking where hurricanes will go, "our skill in intensity forecasting is quite low," says Robert Gall, lead scientist for the US Weather Research Program.
It has long been known that under favorable atmospheric conditions, hurricanes spawn and grow by drawing energy from warm tropical waters in the Atlantic. But "there's a growing recognition that subtle ocean features can have a huge impact" on strength, Dr. Gall says.
Those subtle features, known as warm eddies, are the target of an intense field campaign this season. They constitute one of the National Oceanic and Atmospheric Administration's (NOAA) two priorities for hurricane research this year.
The eddies initially drew researchers' attention in 1995 following hurricane Opal, which underwent an enormous burst in strength shortly before landfall, only to weaken just before it reached the Gulf Coast. Eddies also played a role in the late intensification of hurricane Bret off the coast of Texas last season.
Several researchers looked to atmospheric conditions to explain Opal's sudden changes. But Peter Black, a meteorologist with NOAA, and Lynn Shay, a professor at the University of Miami, looked at the ocean.
Using a satellite called TOPEX/POSEIDON, they found that Opal strengthened as it traveled over a large, deep reservoir of warm water that had pirouetted off a current looping into the Gulf of Mexico. Often covering areas as large as hurricanes, warm eddies emerge from the Caribbean every 11 to 14 months and can reach depths of several hundred feet.
Then, last August, the duo and their colleagues got their first real glimpse of these tropical powerhouses, dropping instruments into an eddy set to peel off a loop current. They found it held six times the heat content of the surrounding water, suggesting an enormous energy reservoir.
"We also made a series of measurements during the rapid intensification of hurricane Bret last year that really surprised us," Dr. Black notes. The team noticed that as it strengthened, Bret passed over a year-old eddy that, from satellite images, looked too weak to worry about. The team took measurements anyway, and found that the dying eddy still held twice the heat content of the surrounding water. Though weak, the eddy held enough heat to drive the storm from a Category 2 to a Category 4 (out of 5).
This year, two eddies remain in the Gulf from last year - in addition to the new eddies that are forming. "It appears that there is a lot more area of deep warm water for any passing hurricane to feed on than last year," Black observes.
Nor are these features limited to the Gulf. Dr. Shay notes that similar eddies spin off from a loop current that covers the straits between Key West, Fla., and Cuba.
Forecasters at the National Hurricane Center now have access to maps the team is developing. The maps relate satellite information on sea-surface heights to the ocean's heat content. Currently, the maps can be updated once every 10 days because only one TOPEX satellite is in orbit. A second satellite is expected to be launched and operating within another year, allowing for more-frequent updates.
Within the next few years, as forecasters have an opportunity to work with the new heat-content information, researchers hope that their approach will replace the use of sea-surface temperatures to gauge changes in storm strength.
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