Right now, the northern Gulf Coast from Louisiana to the Florida panhandle is bracing for more oil from the BP disaster. But powerful, hidden flows of water beneath the Gulf's surface mean other parts of the country could also face the risk of pollution in the coming weeks and months.
A strong, swift ocean current arcs through the southeastern Gulf of Mexico like an enormous upside-down "U." After leaving the Gulf though the Florida Straits, it passes the Keys, Cuba, and the Bahamas, eventually joining the Gulf Stream, which brings warmer waters to the East Coast and Newfoundland before veering westward for northern Europe.
This Loop Current has generally been sitting about 100 miles south of the site of the crippled Deepwater Horizon oil rig, where thousands of gallons of oil are gushing daily into the Gulf of Mexico. But that could change at any time.
"Just how far to the north it extends varies," according to oceanographer Robert Weisberg of the University of South Florida, who studies ocean water circulation. "Sometimes it makes it as far as the wellhead."
Small currents that Weisberg calls "filaments" also spin off from the main current and reach farther northward. That could push oil closer to the coast, or pull it back into the Loop Current and send it heading toward the Atlantic.
Weisberg and colleagues have been using data derived from satellite and flyover imagery, as well as sea surface temperature readings and wind forecasts, to model the likely movements of the visible oil slick. Computer-generated points called "drifters" stand in for particles of oil. "What we're trying to do is simulate not only where particles may go," he says, "but where they were [and] where residual oil may be."
The surface currents move in response to the wind, he notes, and the oil moves with the currents. But "the Loop Current is always there. One thing that we're watching is whether or not oil -- not 'whether or not,' I think it's 'when' -- when oil gets entrained in that current. Because when that happens, oil will move very swiftly to the south, and toward the southern portion of Florida."
It's likely to be weeks before the Loop Current moves firmly into the area of the slick. Weisberg says. But once oil is caught up in the Loop Current, it will reach Florida Straits in about a week, offshore Miami just under a week later, and North Carolina's Cape Hattaras (roughly one thousand miles away) about 10 days after that, he says. At that point, the oil may start to disperse eastward into open ocean, thinning out considerably before it nears the northeastern states.
Sea surface conditions can change quickly in the Gulf. Last week currents seemed to be pushing the oil slick toward the northern Florida coast, while at this writing they are coming from the southeast, moving the slick toward Louisiana's shoreline. Hurricane season is imminent as well, adding more complications.
So Weisberg re-runs the models about once a day using updated wind forecasts, sea surface temperature readings, and aerial data of the oil slick.
The leaked oil poses an enormous risk to Florida, Weisberg says. Surface currents may aim it at the state's northern and western coasts, while the Loop Current will bring it close enough to corals and other sensitive environments along the state's southern and eastern coasts.
"If the winds are not blowing in the right direction for long enough, the oil may simply stay in the Gulf Stream and go on its merry way," he says. But that is a best-case scenario. "Every day that oil seeps up from the wellhead," Weisberg says, "the more of a threat it is to everybody."
Top image from hindcast/forecast model of Gulf oil slick drift, including wind forecast and sea surface temperature data. Oil slick represented as black dots; movement trajectories in magenta. The color contours represent sea surface temperature; superimposed arrows show surface currents. Loop Current is bright orange "U" toward bottom of image. Credit: University of South Florida, College of Marine Science, Ocean Circulation Group