Gulf Spill Oil Driven by Complex Ocean Currents and Eddies

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.

Oil boom stretches along empty beaches, tar balls have washed ashore along the Alabama and Mississippi coasts, and a swirling, oily sheen covers at least 2,500 square miles of the sea surface in the Gulf of Mexico.

So far, currents, winds, and a plume of fresh water flowing into the Gulf from the Mississippi River have acted in concert to hold at bay the oil spewing from a damaged well head 5,000 feet below the sea surface some 40 miles off the Louisiana coast.

In anticipation of the oil’s arrival, some 13,000 people stand ready to combat the spill if it approaches shore, according to the Obama administration. More than a million feet of boom has been deployed. More than half a million gallons of dispersants has been applied.

For anyone using the Exxon Valdez spill in Alaska’s Prince William Sound as a visual reference point, it might look as though the Gulf spill so far is a dodged bullet.

But the differences between the two events are significant, cautions Michelle Wood, a marine biologist who recently became head of the ocean chemistry division of the National Oceanic and Atmospheric Administration’s Atlantic and Oceanographic and Meteorological Laboratory in Miami. Not the least of those differences is the seascape into which the oil is flowing.

Gulf Spill Unlike Exxon Valdez

The Exxon Valdez spill involved a large, single, intense pulse of oil into Prince William Sound – “a shallow, near-shore environment with a rocky coast,” she explains. The heavy crude had lots to cling to as it came ashore. In the Gulf, “spill” is a so-far continuous infusion of a lighter grade oil, which at least initially forms a foamy mousse rather than tarry blobs. And so far, the oil has remained far at sea.

The apparent gap between preparations for the oil’s arrival along the Gulf Coast and its behavior so far testify to the complex marine environment the oil enters as it spews from the broken well head, researchers say.

The system is chaotic enough that given enough time, say 90 days, oil in some form could wind up anywhere from the Mexican Coast to Palm Beach, research suggests.

“We call it a mini ocean,” says Steven DiMarco, an oceanographer at Texas A&M University in College Station, Texas. “Many of the processes that occur in the Gulf of Mexico occur in the much larger basins like the Atlantic and the Pacific.”

Atlantic “Conveyer Belt”

The main oceanic feature is the so-called loop current, essentially the Gulf’s section of a much longer current that forms the Atlantic Ocean’s so-called conveyor belt.

The belt, which drives warm tropical waters north toward Greenland, where it sinks and cools, begins in the equatorial Atlantic off Brazil. The current snakes into the Caribbean and then north between Cuba and Mexico’s Yucatan Peninsula.

The volume of water moving through the Yucatan Straight is so enormous and travels with such speed – essentially at the pace of a brisk walk – that it forms a loop that meanders north of Cuba, then makes a U-turn southward toward the island before heading out through the Florida Straights to form the Gulf Stream.

Below about 1,000 meters (3281 feet), however, the regime shifts.

Circulation runs counterclockwise as seawater spills over a sill spanning the Yucatan Straight. In a kind of watch-your-step plunge, water flows over the sill and into the deepest reaches of the Gulf. It travels east until it reaches the continental shelf off Florida’s west coast. But the sill across the Florida Straight is far shallower, forcing the deep flow to ricochet back toward Texas and Mexico. At these depths, the current moves more than 100 times slower than surface currents.

As oil from the Deepwater Horizon blow-out rises, it would encounter swirling eddies that spin off these broader flows, distributing some of the oil horizontally, says Arthur Mariano, an oceanographer at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science on Miami’s Key Biscayne.

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.

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.

IN PICTURES: Louisiana oil spill