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.