Gulf 'Loop' Currents Intensify Hurricanes

Gulf Currents That Turn Storms Into Monsters - New York Times (registration required) outlines the role of 'loop' currents in increasing the strength of hurricanes as they pass through the Gulf of Mexico. Loop currents are streams of deep, warm water that enter the Gulf between Cuba and Mexico and exit through the Florida passage. They can cast off eddies that remain in the Gulf for months. This warm water helps provide energy to a hurricane.
Be sure to check out the graphic in the article. It contains an overview of the paths of Katrina and Rita as they strengthen and pass over or near this current.