Gulf spill oil driven by complex ocean currents and eddies

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.

IN PICTURES: Louisiana oil spill

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.