Hurricanes Impact

Natural Hazards


Gulf Warm-Water Eddies Intensify Hurricane Changes

National Science Foundation Press Release 05-174

depth of 26 degree isotherm, description follows
Click image to enlarge.

New Loop Current warm-water eddies, shown in orange, recently formed in the Gulf of Mexico.

October 3, 2005 Scientists monitoring ocean heat and circulation in the Gulf of Mexico during Hurricanes Katrina and Rita have a new understanding of how these tropical storms can gain intensity so quickly: The Gulf of Mexico's "Loop Current" is likely intensifying hurricanes that pass over eddies of warm water that spin off the main current.

"A positive outcome of a hurricane season like this is that we've been able to learn more about the Loop Current and its associated warm-water eddies, which are basically hurricane intensity engines," said Nick Shay, a University of Miami Rosenstiel School of Marine and Atmospheric Science (RSMAS) meteorologist and physical oceanographer.

The Loop Current is a horseshoe-shaped feature that flows clockwise, transferring warm subtropical waters from the Caribbean Sea through the Yucatan Straits into the Gulf of Mexico.

This year, the Loop Current extended deep into the Gulf of Mexico during hurricane season. Currents at this time of year typically become unsteady and pinch off deep, warm eddies, said Shay. The warm water then becomes ideal for hurricanes in the process of intensifying.

"Scientists have known that hurricanes form above the world's warmest ocean surface waters," said Jay Fein, program director in the National Science Foundation (NSF)'s Division of Atmospheric Sciences, which funded the research. "This study adds new information about hurricanes' journeys to landfall, and will help to better predict their paths and intensity changes during their final hours over open water."

After Hurricane Katrina and a week before Hurricane Rita, Shay, Peter Black from the NOAA's Atlantic Oceanographic and Meteorological Laboratory (AOML) and Eric Uhlhorn of the University of Miami/NOAA Cooperative Institute for Marine and Atmospheric Science, deployed Airborne Expendable Conductivity, Temperature and Depth profilers (AXCTDs); Current Profilers (AXCPs); and Bathythermographs (AXBTs) to obtain information on water temperature to depths of up to 3,300 feet (1,000 meters).

The AXCTDs and AXCPs, which were funded by NSF, are dropped from aircraft and measure salinity and currents.

Meteorologists are learning a great deal as hurricanes pass over the deep, warm waters of the Loop Current, Shay said. "We have long been aware that these currents are an important way for the ocean to distribute heat and energy, but until now, we just didn't have much data on the role they played in building hurricane intensity."

Two days before Hurricane Rita, Black and Rick Lumpkin of AOML, and Peter Niiler of the Scripps Institution of Oceanography, deployed surface drifters that measure surface and subsurface thermal conditions while traveling clockwise around a Loop Current warm eddy just south of La. The eddy was lying in the path of Rita.

"This represents one of the most comprehensive ocean-data sets where two major hurricanes passed through the same region," said Frank Marks, director of NOAA's Hurricane Research Division. "This series of observations is a testament to how new ocean observations are helping us understand hurricane intensity changes."

"The last time there was a season with two Category 5 hurricanes in the same basin was in 1961 with Carla and Hattie," said Shay. "However, the same phenomenon occurred the year before in 1960 with Donna and Ethel."

Hurricanes Katrina and Rita also have much in common with Hurricane Opal, a category 4 storm that occurred a decade ago, on Oct. 4. During Opal, meteorologists first recognized the pivotal role that deep, warm eddies play in quickly building hurricane intensity. Opal encountered a warm- water eddy in the Gulf of Mexico and strengthened in intensity from Category 1 to Category 4 in just 14 hours.


Media Contacts Cheryl Dybas, NSF (703) 292-7734 Ivy Kupec, U. of Miami/RSMAS (305) 421-4704 Jana Goldman, NOAA (301) 713-2423 Cindy Clark, UCSD/SIO (858) 534-3624

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CU-Boulder Researchers Chart Katrina's Growth
In Gulf Of Mexico

Press Release Sept. 15, 2005

hurricane katrina sea surface heights and max wind speeds in miles per hour, description follows
Click image to enlarge.
The loop current is shown in red and orange.

An image produced by a University of Colorado at Boulder team shows the wind speeds of Hurricane Katrina increasing dramatically as it passes through the warm waters of the Gulf of Mexico's Loop Current toward the Gulf Coast in late August.

Processed at CU-Boulder's Colorado Center for Astrodynamics Research, or CCAR, the image was produced with data from four satellites that use altimeters to measure sea-surface height to an accuracy of less than 2.5 cm (1 inch). The researchers extrapolated ocean temperatures from subtle height changes in the water measured by altimeters bouncing microwave pulses from the satellites to the ocean surface and back, said aerospace engineering Associate Research Professor Robert Leben of CCAR.

The sea-surface height image shows the warm Loop Current standing 0.51 to 0.76 meters (20 inches to 30 inches) higher than the surrounding water, said Leben.

As Katrina passed over the Loop Current, it gained a huge amount of energy from the unusually warm waters that subsequently increased its maximum winds, said Leben. The storm evolved from a Category 3 hurricane to a Category 5 hurricane in just nine hours by converting heat from the Loop Current into energy, he said.

"A hurricane is like a steam engine," said Leben. "The more heat that is put into it, the faster it is going to run. When I saw the predicted storm track over the Loop Current, I became concerned we might see a doomsday scenario."

In the Gulf of Mexico, there is a tight correlation between the sea- surface height measured by the satellites and the temperature of the waters, said Leben. "The higher the sea-surface is above the mean, the deeper the warm water underneath it," he said.

According to CCAR Director George Born, an aerospace engineering professor who works with Leben, the conditions in the Loop Current were unusual. "Ordinarily, the Loop Current does not intrude this far north and west into the Gulf," Born said. "If it had not, the storm would not have gained such intensity."

The CCAR researchers used data from the U.S./French TOPEX/Poseidon and Jason-1 satellites, as well as the U.S. Navy's Geosat Follow-On satellite and the European Space Agency's Envisat satellite. They then combined the data with hurricane wind speed and position data from the National Oceanic and Atmospheric Administration to produce the unique image, Leben said.

More than 100,000 images from the CCAR Web site, which is maintained primarily by CU-Boulder undergraduates, were downloaded last year by federal agencies, private customers and the public, said Born. The center produces daily maps of changing ocean currents around the world for governments and private businesses.

The CCAR Web site for mapping ocean currents and eddies in the Gulf of Mexico is funded primarily by NASA's Jet Propulsion Laboratory and is used by petroleum corporations as well as educators and the general public, said Leben.

Since May 2005, satellite ocean-current maps generated by CCAR have been used to support a bluefin tuna tagging program by New Zealand's Ministry of Fisheries, helping scientists locate prime tuna habitats for a tag-and-release program. The CCAR maps illuminate "cold-core" eddies where the fish congregate, helping the government better understand the relationship between sea-surface heights and the distribution and migration of the tuna, Leben said.

The group also has been providing ocean-current maps in the Gulf of Mexico to help marine-mammal researchers locate and count sperm whales. The sperm whales often congregate in nutrient-rich eddies near the edge of the Gulf's Loop Current where squid - their primary food source - are plentiful, Leben said.

Two years ago CCAR ocean-current maps helped a private oil company find a suitable towing route through the Gulf of Mexico for a submersible drilling rig used in deep water oil and gas exploration. The company avoided problematic eddy currents and took advantage of beneficial eddies, reducing transit time by 50 hours on the 400-mile journey and saving the company $650,000 in rig downtime and towing costs.

CCAR faculty and students focus on research in astrodynamics, satellite meteorology, oceanography, geodesy and terrestrial vegetation studies.


Contact: Robert Leben, (303) 492-4113 George Born, (303) 492-8638 Jim Scott, (303) 492-3114

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