The worst oil disaster in the history of America was unleashed following a deep sea explosion which toppled the Deepwater Horizon, an oil drilling rig in the Gulf of Mexico, off the coast of Louisiana. Some estimates are that as much as 40,000 barrels of oil a day have gushed from this hole at the bottom of the sea, threatening birds, fish and other wildlife and fouling numerous delicate ecosystems not just along the coast line, but deep beneath the surface of the ocean.

As oil and gas continues to gush from the Deepwater Horizon/British Petroleum oil leak in the Gulf of Mexico, oceanographers and modelers are digging deep into their experiences and skills to try to understand and analyze the dispersion and fate of the massive amounts of hydrocarbons spreading throughout the Gulf and possibly into the Atlantic Ocean. Although the extended, convoluted tongues of surface slicks, clearly visible in satellite, aerial and kite images are the most obvious indication of the extent of the leak, most of the residue lies far below the surface and is propelled along by deep ocean currents, rather than the surface winds. The oil/gas/hydrates mixtures emanating from deep inside the wellhead are initially driven upwards by the subsurface gas pressure. Methane hydrate ice deposits (a methane/water compound located extensively over the deep slope regions of the world’s oceans) are normally frozen at high pressures and temperatures less than 4°C. As the slush rises and melts, methane gas bubbles up through the water column and is slowly released into the atmosphere. Methane is a powerful greenhouse gas, some 20-30 times more potent as CO₂ (although it has a relatively short half-life of about 10 years).

As the rising plume fractionates into various constituents, each with its characteristic density, multiple plumes will form into various layers of contaminated ocean water each with a density matching that of the surrounding water (sea water density monotonically increases with depth, owing to the combined effects of pressure, temperature and salinity), they become neutrally buoyant. One deep, abyssal plume is being monitored and analyzed by researchers at the University of Georgia. They have found extensive plumes whose upper surface is about 1,100 m below the sea surface, 5 km wide and as thick as 460 m. Oceanographers from the University of South Florida have found an even more extensive plume stretching northeast of the blowout with the contaminants forming into two separate layers; one centered at 365 m below the surface and the second at about 950 m. Scientists now believe that up to 90% of the hydrocarbons lie well beneath the surface.

So how to track the movement of the plume(s) so far below the surface, well out of site of any satellite technology and very expensive to monitor from a slowly moving ship and much too deep for patrolling submarines? This is where modern numerical modeling of ocean circulation and mixing is proving to be crucial. Three dimensional ocean models solve the coupled non-linear equations of momentum, continuity and the equation of state (relating ocean density to temperature, salinity and pressure) for a digitized three-dimensional ocean where it is necessary to resolve ocean bathymetry and features down to 10’s of kilometers in the horizontal and tens of meters in the vertical. Boundary conditions involve applying realistic wind stresses to the surface waters, and an understanding and quantification of incoming flows through the open boundaries such as the Yucatan Strait. Modeling the Gulf of Mexico benefits from its being classified as a semi-enclosed sea, with its major connection to the Caribbean Sea being the Yucatan Strait or Channel and to the Atlantic Ocean through the Straits of Florida.

The strong Loop Current which flows into and out of the Gulf is classified as a western boundary current. In 1947, Henry Stommel of the Woods Hole Oceanographic Institution first recognized the essential role the latitudinal variation of the Coriolis force plays, along with the location of the major wind belts (Trades and the higher-latitude Westerlies) in establishing and driving these intense ribbons of rapidly flowing waters. Western boundary currents are found along the western boundaries of the major ocean basins (e.g., the Kuroshio Current in the northwestern Pacific, the East Australian Current in the southwestern Pacific, The Brazil Current in the southwestern Atlantic Ocean and the Gulf Stream in the northwest Atlantic). The Loop Current is derived from the Caribbean current which flows through the Caribbean Sea, becoming the Yucatan Current as it enters the Gulf of Mexico between the Yucatan Peninsula and Cuba. The Loop current has many interesting and important characteristics.

Bulging into the Gulf of Mexico like a giant aneurysm, huge clockwise-rotating anticyclonic eddies periodically break off into Loop current eddies (see Fig. 1) every 6 to 12 months. These eddies contain vast amounts of heat and kinetic energy and are typically penetrate down to 600 m. since they are comprised of lower density tropical water, the eddies are buoyant and are easily detected from satellites by their temperature, color and surface elevation (they rise higher than the surrounding gulf water by 10’s of cm).

Hurricane Katrina passed over a Loop Current eddy before it smashed into New Orleans in the summer of 2005, sucking heat out of the eddy and fueling a category-5 hurricane before making landfall. The Loop Current eddies once disentangled from the Loop Current itself, then slowly drift westwards for several months before colliding with the western continental shelf of the Gulf. So a detailed understanding of the nature and dynamics of the Loop Current and its eddies is fundamental to understanding how deep contaminants will be transported away from the spill wellhead site and eventually mixed into the world ocean. Unfortunately, at present, it is impossible to predict the full extent and long term consequences of this disaster.