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ONE WORLD: Hurricanes and the Loop Current in the Gulf of Mexico

| posted on July 22, 2010

If you’ve ever surfed in the Gulf of Mexico during the summertime, you’ve probably noticed that the seawater from Galveston to Pensacola to Tampa Bay is, in a word, hot. So hot in fact, that on an August day, a dip in the near-90-degree ocean offers little relief and can melt the wax right off your surfboard. Hot water near the surface of the ocean can give rise and continuing fuel to tropical storms and hurricanes, but it takes a special set of circumstances for storms to reach the truly awesome scale of hurricanes Katrina and Rita. In the Gulf this past summer, these two storms passed over an oceanic anomaly that scientists have taken to calling the Loop Current. This meandering column of Gulf Stream water can be nearly 80 degrees Fahrenheit and can extend as deep as 300 feet below the ocean’s surface — providing nearly limitless fuel and preventing cold-water upwellings. This in turn, can allow a storm like Katrina or Rita to grow from merely damaging to truly cataclysmic in the span of a day.

In the past, loop currents were of more interest to fishermen and shipping interests in the Gulf and other warm-water parts of the world, as they could dramatically affect schools of fish and local ocean weather conditions. Recently however, they have become a focal point for hurricane research.

According to experts at the National Science Foundation, the warm, clear Gulf Stream waters that flow through the Gulf of Mexico tend to get diverted as they pass from the Yucatan Peninsula up toward Cuba. This creates a horseshoe-shaped column of water up to 100 times as large as the Amazon River that flows into the Gulf of Mexico before either passing out between the Florida Keys or becoming a circular eddy current that cuts off from the Stream and can linger in the Gulf for months before finally cooling down. In 1999, Nick Shay, a scientist at the University of Miami, identified this loop current and its accompanying 330-foot-deep “warm core rings” as the energy source likely to cause a hurricane to grow from a Category One to a Five so quickly. “This is the heat,” Shay said. “This is the energy source. It’s like a big fuel injector in the middle of the ocean.”

While warm water fuels hurricanes, the truly hot stuff normally reaches down only about 120 feet. As a storm passes over this warm water, it typically draws up the heat, and causes deeper, colder upwellings as it rolls along. This upwelling can weaken a storm, preventing it from gaining too much strength. But when a storm encounters one of the 75-mile-wide-plus warm core rings, it can grow explosively, feeding on a nearly inexhaustible supply of deep-water heat. Shay said that with better oceanic temperature modeling, forecasters should soon be able to predict whether a storm will encounter one of these loops. “Ultimately,” he 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.”

In 2005, Shay and other scientists studied the Gulf Loop Current, and watched in dismay as Katrina in particular followed a broad, deep current all the way to New Orleans. Rita initially took a similar course, feeding off deepwater heat, but eventually turned off the loop current’s trajectory and weakened to a Category Three storm as it made landfall. In the wake of Katrina, and before Rita, University of Miami and NOAA researchers dropped measuring devices into the Gulf of Mexico that they say will help give them a better handle on loop currents and their unpredictable effects. Shay said that if there was any silver lining to all this year’s destruction it would be in advanced prediction skills for the future. “A positive outcome of a hurricane season like this,” he said, “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.”

The Gulf of Mexico is not the only place these warm-water currents occur. During the summer, a Gulf Stream eddy will often break off near Maine. Seven other areas in the world, including spots south of Japan and Indonesia, allow for deep enough energy to create major hurricanes, also known as cyclones or typhoons. Other conditions like a very still upper atmosphere and considerable humidity in a storm’s vicinity must also be present to fuel a major hurricane. As this year’s hurricane season enters the final stretch, keep an eye on this page from NOAA (http://www.aoml.noaa.gov/phod/cyclone/data/go.html) for deep-water ocean temperatures in the path of an advancing storm. That, and the buxom blonde on the Weather Channel could give you the best idea of whether to grab your board — or run for the hills.