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By Edwin Schiele
May 21, 2010

On April 20, 2010, an explosion took place onboard the Deep Water Horizon, a drilling rig operated by British Petroleum in the Gulf of Mexico 50 miles off of the Louisiana coast, killing eleven workers. The rig had just completed drilling a new well into the seafloor 5,000 feet below the ocean surface. Two days after the explosion, the rig sank. Oil was discovered gushing out of the broken pipes. Procedures set in place to plug these leaks failed, and oil has flowed out largely unchecked.

Once in the ocean, the oil is subject to the whims of winds and ocean currents. Officials at NOAA and other agencies face the challenge of predicting the spill’s trajectory. The Gulf is home to a large fishing industry as well as endangered marine animals such as sea turtles and sperm whales. It is surrounded by sensitive coastline that includes marshes and beaches. There are reports that by mid May, heavy oil had penetrated Louisiana’s wetlands. Good forecasts require a combination of knowledge of past current behavior, timely surveillance of the surface currents, accurate weather forecasts, and models that can translate these data into future projections. Because oil spills occur on a weekly basis in the Gulf, although at much smaller scales, officials at NOAA and other agencies that track spills have had plenty of opportunities to hone their skills.


Seasonal Wind Patterns



Robert Hetland, an associate professor in the Department of Oceanography at Texas A & M University, tracks fresh water from the Mississippi and Atchafalaya Rivers as it enters the Gulf and mixes with the salt water. He is therefore familiar with the currents along the Gulf Coast. To trace the currents, he uses the Regional Ocean Modeling System, an open source model (anybody can use it) built around fluid dynamics equations. Researchers such as Hetland can adapt it for different regions and scales. Hetland provides information that the model generates to NOAA as well as the Texas General Land Office, the state agency that responds to oil spills.

Hetland says that there are two seasonal wind patterns along the Gulf Coasts that can impact where the oil ends up. From September into May, the winds generally blow westward off of Louisiana, and the currents tend to hug the coast. Such currents have brought fresh water from the Mississippi as far as Mexico, so oil could potentially follow that same path.

During the summer, the winds tend to blow onshore and slightly east. Hetland says that fresh water from the Mississippi tends to pool off the Louisiana coast. This weather pattern would likely keep the oil offshore.

Hetland cautions, however, that these seasonal patterns only represent general trends. Winds and the surface currents they drive can switch course at any time. Just as one can expect warm spells in the middle of winter, currents can switch to any direction during any season.


The Loop Current as it flows into the Gulf Stream

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(click on the arrow to play high resolution version)

The Loop Current and the Gulf Stream - The animation focuses on the Loop Current as it flows into the Gulf Stream (a major surface current). Note the black colors indicate the warmest ocean surface temperatures and and light blues indicate the coolest temperatures. Sea surface temperature (SST) simulation from the Geophysical Fluid Dynamics Laboratory's (GFDL) high resolution coupled atmosphere-ocean model.
Credits: Anthony Rosati (GFDL) and Chris Kerr (GFDL), Princeton University.

Complicating the forecasts further, the oil spill occurred during the transition from the non-summer to the summer season.

The largest feature in the Gulf is the Loop Current. This powerful current is fed by warm tropical water flowing north between Cuba and the Yucatan Peninsula. Upon entering the Gulf, it heads north then loops clockwise south before turning east through the Florida Straights. At that point, called the Florida Current, it turns north up Florida’s east coast and becomes the Gulf Stream.

Scientists fear that if the oil gets entrained into the Loop Current, it could shoot up through the Florida Keys and up the east coast, threatening sensitive coastlines from Miami to Cape Hatteras.

The Loop current’s path through the Gulf is notoriously difficult to predict. But scientists have observed a general cycle. At the start of the cycle the tip of the loop sits north nearly to the Florida Shelf, although the extent of its reach varies. The top part of the loop then shears off and forms a clockwise swirling eddy. At first, the eddy simply hangs around, reattaching then detaching three or four times from the loop. Finally the eddy drifts off, usually to the southwest, completing its separation. (The oil industry, which monitors these eddies due to their potential impact on drilling operations, has provided these eddies with creative names such as Fast Eddy, Eddy Murphy, and Eddy Haskell. They also have named some after famous oceanographers such as Eddy Ekman, which separated last fall.) Once the eddy has left, the remaining stubble of the Loop Current retreats south. The loop eventually moves back north, another eddy shears off, and the cycle begins again.

At the time of the spill, the loop had penetrated farther north than usual, just south of the oil patch. By May 19, NOAA had reported that small amounts of oil had been entrained into the current, although not enough to cause concern. As the season shifts to summer and the winds push the oil south towards the current, many scientists and officials fear that significantly more oil will likely enter the current.

How much oil depends how long the loop stays north. When a new eddy separates and the loop retreats south, it will be out of reach of the spill.







Making sense of this Loop Current cycle has been the focus of Robert Leben at the Colorado Center for Astrodynamics Research at the University of Colorado. He uses satellite altimetry data dating back to 1993 to trace the current’s changing positions and calculate its flow. Satellite altimetry measures sea surface height and that can be used to produce topographical maps of the sea surface. The paths of geostrophic currents such as the Loop Current are tied into the balance between the Coriolis effect, which steers the water towards the center where it piles up, and the resulting pressure gradient in the center that pushes the water back downhill. Leben determined that the continuous 17-cm contour on the sea surface height maps could represent the core of the current.

Using altimetry data has some advantages. Unlike sea surface temperature maps, which highlight the warm tropical water flowing through the current, the satellites can gather data on cloudy days. The ocean topography maps also enable Leben to calculate the flow rates. Altimetry, however, does not provide real time information and the resolution of the maps is not as fine as sea surface temperature images. It is therefore not useful for making short-term forecasts and tracking the oil spill.

Leben says that unraveling the mechanisms that controls the back-and-forth movement of the loop current has proven difficult. The times between eddy separations range from 4 to 18 months. There is no apparent seasonal pattern. Even the speed and distance that the loop penetrates north and retreats south varies from cycle to cycle. At the time of the spill, the loop had penetrated farther north than usual. Scientists have looked at the Yucatan Current, which feeds into the Loop Current, for clues, but Leben says that nothing conclusive has been observed.

Leben, however, has uncovered some statistical correlations based on the previous behaviors of the current that so far have proven successful in predicting the length of the cycle. In essence, the further south Loop Current retreats after an eddy separates, the farther north the loop will rebound and the longer the period of time before the next eddy separates.

The last eddy, Eddy Ekman, separated from the current in September, 2009. The Loop Current then retreated farther south than usual. Leben predicts that if past correlations hold, the next eddy won’t separate and the current won’t retreat south until late fall or early winter of 2010. Leben predicts that the loop will therefore stay within range of the oil spill through the summer and fall with the potential of picking up and transporting significant quantities of oil.

Patterns in ocean currents, however, don’t always hold. Observers of the Loop Current learned that fact first hand when they tracked the separation of Eddy Zorro in 2007. Instead of retreating to the southwest, Zorro moved north into the oil lease area, then became completely reabsorbed into the Loop Current.

So even with all of the observational powers scientists now possess and strides they have made in deciphering the ocean’s patterns, scientists are still must wait and hope for good luck; that the winds keep the oil offshore, and the Loop Current retreats sooner than expected.