Gulf Warm-water Eddies Intensify Hurricane Changes

"A positive outcome of a hurricane season like this is that we've beenable to learn more about the Loop Current and its associated warm-watereddies, which are basically hurricane intensity engines," said NickShay, a University of Miami Rosenstiel School of Marine and AtmosphericScience (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 theYucatan Straits into the Gulf of Mexico.

This year, the Loop Current extended deep into the Gulf of Mexicoduring hurricane season. Currents at this time of year typically becomeunsteady and pinch off deep, warm eddies, said Shay. The warm waterthen becomes ideal for hurricanes in the process of intensifying.

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

After Hurricane Katrina and a week before Hurricane Rita, Shay, PeterBlack from the NOAA's Atlantic Oceanographic and MeteorologicalLaboratory (AOML) and Eric Uhlhorn of the University of Miami/NOAACooperative Institute for Marine and Atmospheric Science, deployedAirborne Expendable Conductivity, Temperature and Depth profilers(AXCTDs); Current Profilers (AXCPs); and Bathythermographs (AXBTs) toobtain 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 overthe deep, warm waters of the Loop Current, Shay said. "We have longbeen aware that these currents are an important way for the ocean todistribute heat and energy, but until now, we just didn't have muchdata 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, deployedsurface drifters that measure surface and subsurface thermal conditionswhile traveling clockwise around a Loop Current warm eddy just south ofLa. The eddy was lying in the path of Rita.

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

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

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