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Gulf spill oil driven by complex ocean currents and eddies

The BP oil spill in the Gulf of Mexico is far different than the Exxon Valdez spill in Alaska. The complex marine environment has currents and eddies that could carry the oil anywhere in the Gulf.

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Once it reaches the surface, currents and ever-shifting winds can carry the oil just about anywhere, adds Peter Niiler, a researcher at the Scripps Institution of Oceanography in La Jolla, Calif., who has conducted extensive studies of current patterns in the Gulf.

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A five-year experiment

In a five-year experiment in the late 1990s, Dr. Niiler and a colleague dropped between 700 and 800 drifters – devices to help track currents – into the Gulf at locations where offshore drilling was taking place.

Within 90 days, the drifters could be anywhere in the Gulf, including Mexico or even as far away as Miami Beach, he says.

Of key concern is the loop current, which at the moment is brushing the southernmost reaches of the spill. Oil can be swept up and carried along as the current moves through the Florida Straight and out into the Gulf Stream.

By that time, the oil is likely to have been highly diluted and dispersed, Mariano says. Oil on the surface weathers, losing the lighter chemicals it contains to evaporation. As it does, it grows denser, then sinks.

But periodically, the loop current's loop stretches far enough north that it becomes vulnerable to a kind of self-pruning; it pinches off to become a loop current eddy. These eddies are roughly 60 miles across, and once they break free, they head west. They can break off on average every three to 17 months. Oil can become entrained in the eddy and travel toward Texas and the Mexican coast.

Loop currents and eddies

"Knowing were the loop current is, knowing whether a loop current eddy is going to break off, and knowing where that eddy is going to go" is vital to understanding where some of the oil may be headed, Dr. DiMarco says.

Indeed, Niiler points to a large eddy currently on the loop current's eastern flank as a potential pruning tool that could trigger the formation of a loop-current eddy.

On Saturday, the Coast Guard and the EPA approved BP's used of dispersants at depth to try to break up the oil as it leaves the well head. But it's unclear how well that works, researchers say.

The effort "in some sense is one big experiment," Mariano says.

Over the short term, Mariano says he expected the oil to collect on the surface and at the bottom. But as the surface oil weathers, it will sink to join oil at the bottom as well. Given the circulation patterns at depth, remaining oil will be patchy. But "a lot of that oil at the bottom is going to hang around for a long time," he says.

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