Nov. 1, 1999 VIRGINIA KEY, FL -- Just hours from the United States coastline, Hurricane Opal suddenly strengthened. Its winds shot up from 110 mph to 135 mph in a mere 14 hours. Similar sudden and unexpected intensification just before landfall happened with Hurricanes Allen and Camille -- leaving barely enough time to warn people, and almost no time to evacuate.
Now, a University of Miami Rosenstiel School of Marine and Atmospheric Science professor has identified the probable cause of those sudden intensifications and, perhaps more importantly, mapped some of the hot spots where this season's hurricanes are likely to strengthen dramatically just before landfall.
The cause, UM associate professor of Meteorology and Physical Oceanography Lynn K. "Nick" Shay said, are "warm core rings," where warm water extends down to a depth of 100 meters or more.
The discovery, which is the result of a joint effort between Shay and Peter Black of NOAA's Hurricane Research Division, could save lives and millions of dollars spent on unnecessary evacuations by helping more accurately predict how powerful a storm will be when it strikes land.
"This is the heat. This is the energy source," Shay said. "It's like a big fuel-injector in the middle of the ocean."
Warm ocean temperatures fuel hurricanes. Normally, though, surface temperatures of 26 degrees centigrade or higher only extend down about 30 to 40 meters. A passing storm draws some energy from the warm water as it passes, but it also stirs it, mixing it with cooler water from below and lowering the temperature of the surface water. The now-cooler surface water then provides less energy for the storm, keeping it from intensifying much further.
In a warm core ring, however, the warm water goes much deeper. It doesn't cool that much when a storm passes because it doesn't mix with cooler water from below. A hurricane passing over one of the rings, which are 180 to 220 kilometers in diameter, gets a rich, deep source of energy that, coupled with the right atmospheric conditions, can suddenly turbo-charge a hurricane and turn a minimal storm into a monster.
"Ultimately," Shay said, "what we are aiming for is to be able to say that when it encounters this ring, you may be looking at a category four or five storm."
A warm core ring is forming right now in the north Gulf in a position to affect passing hurricanes this season. Like the ones discovered before it, the ring was predictable. The rings develop every 11 to 14 months at the top of a warm-water current that loops up into the Gulf of Mexico from the Caribbean Sea. Then, typically, they drift westward at one to four kilometers per day over a period of several months until they break up along the Mexico or Texas coast. Another one forms in the northern Atlantic, off the coast of Maine.
Shay and Black plan to map a detailed three-dimensional grid of the ring in the Gulf. They also are using historical hurricane tracking information to determine whether warm core rings could account for the sudden intensification of past storms.
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