NOAA 2000-521
Contact: Maureen O'Leary


As the 2000 hurricane season enters it's second half, NOAA's Hurricane Hunters are taking another look at the Gulf of Mexico's hidden hurricane fuel tanks, two eddies of deep warm water spanning 15-20 percent of the Gulf, and their parent Loop Current. And there is good reason to do so, the peak season for hurricane development in the Gulf of Mexico falls in the last weeks of September and the first weeks of October.

Since 1998, Peter Black of NOAA's Hurricane Research Division and Lynn Shay of the University of Miami have researched these eddies and their potential for turbo- charging any hurricane that may cross their path. Black and Shay are studying how such deep pockets of warm water provide enough heat energy for a hurricane to rapidly intensify from a minimal hurricane to a monster in a matter of hours. Rapid intensification of hurricanes is still a crucial challenge in hurricane forecasting.

The Loop Current is a stream of warm Caribbean water that enters the Yucatan Straits, meanders northward, sometimes extending to the Gulf Coast, and exits into the Florida Straits after a sharp turn around the Florida Keys where it becomes the Florida Current. Deep warm water eddies in the Gulf are spun off from the Loop Current as it goes through its 10-14 month cycle.

Warm eddies, like the two present in the Gulf this summer, are a tremendous source of energy to a storm that crosses their path, say Shay and Black. The hurricane winds draw heat from the water to fuel the storm, mixing the warm waters with the cooler waters below as the storm passes by. Because the layer of warm water is so thick within the Loop Current and eddy, the ocean surface experiences less storm-induced cooling than it would outside the these features, allowing for further intensification of the storm. Black and Shay believe that last year's Hurricane Bret, which jumped from a Category 2 to a Category 4 in just a few hours, may have drawn its energy from another eddy remnant off the coast of Texas. Any warm ocean feature close to shore could allow for rapid intensification of storms just before landfall.

In preparation for Hurricane Gordon and later Tropical Storm Helene moving into the Gulf, Black and Shay considered two major features that the storms would likely pass over: a large eddy currently in the central Gulf, just south of the Mississippi delta, and the Loop Current itself. These features were selected for study based on satellite and in situ measurements.

On September 13, the scientists observed the pre-storm structure of the Loop Current and adjacent eddies during a nine-hour flight in one of NOAA's WP-3D airplanes. Data gathered aboard the aircraft analyzed heat content before and after Gordon and Helene tracked over the Loop Current. Data obtained from ocean profiling instruments dropped from the P-3 showed that the eddy south of Louisiana is significantly weaker than previous data indicated. Therefore the focus this season is on the Loop Current itself, and Black and Shay are fortunate that both Gordon and Helene passed over the current. Each acted as expected, strengthening while over the warmer water, and then weakening over the cooler waters of the continental shelf. Even after these two storms passed, the heat content in the Loop Current was well above the threshold needed to sustain a storm. This is vitally important to know for intensity change in storms approaching landfall.

The ocean is not the only factor in hurricane intensity change. NOAA hurricane researchers also consider environmental data gathered around the storm by NOAA's Gulfstream-IV jet, a surveillance aircraft operated by NOAA's Aircraft Operations Center. Though the G-IV missions are designed to improved track forecasts for landfalling hurricanes, an added benefit is measurement of the wind shear and environmental moisture and stability that can affect intensity. Wind shear is the difference in wind velocity at upper and low levels in the atmosphere. High wind shear, dry air, and low stability are associated with weakening of tropical storms. Direct measurements of these variables is unavailable over the Gulf of Mexico without use of the Gulfstream jet.

"We hope to discover the relative importance of the loop current and environmental wind shear in determining hurricane intensity change," Black says.

The data gathered suggest that much like major hurricanes Opal and Bret, the ocean represents a fuel-injector to the atmosphere for all types of tropical cyclones, even tropical storms and minimal hurricanes.

Black and Shay hope that data gathered during this study will enhance knowledge and predictability of major hurricanes, which translates into improved intensity forecast, increased warning time, and better preparedness in coastal regions.

NOAA's mission is to describe and predict changes in the Earth's environment and to observe and wisely manage the Nation's coastal and marine resources.