A frame from an animation showing the possible route into the Atlantic Ocean of oil and dispersant from the spot of the Deepwater Horizon spill in the Gulf of Mexico.
Within days of the Deepwater Horizon oil well blowout on April 20, 2010, Los Alamos National Laboratory (LANL) oceanographer Mathew Maltrud, working at the New Mexico Supercomputing Applications Center, ran a simulation of the spilled oil and sent a visualization to collaborator Synte Peacock of the National Center for Atmospheric Research (NCAR).
Maltrud and Peacock had been studying – and continue to study – ocean dynamics and the relationship between the ocean, climate, the environment and Earth’s atmosphere. All of the gushing oil was at once fascinating and fearful, and it piqued their curiosity.
“Even very experienced ocean modelers looked at the speed of the ocean currents in the simulations and thought, ‘Man, we forget how fast the ocean can move sometimes.’” says Maltrud. “Most of us are working on climate time scales; we’re not used to thinking in terms of weeks to months. It was very impressive in terms of speed.”
About the same time, the two heard from Peacock’s former Ph.D. adviser and occasional collaborator Martin Visbeck, who lives in Germany. Visbeck mentioned that a low-grade panic was sweeping parts of Europe, where citizens began to fear the oil might someday pollute their coasts. With visions of the Alaskan Exxon Valdez spill still fresh after 20 years, many were wondering when Gulf oil might reach their continent and how diluted might it be. Visbeck told his German colleagues of his friends who had a global model that might provide some answers. Suddenly, real-time events were driving science at a frantic pace.
The U.S. Department of Energy allotted Maltrud generous computing hours on the Jaguar Cray XT system at Oak Ridge National Laboratory’s Leadership Computing Facility. He began exhaustive weeks programming a series of ensembles, each a varied group of simulations in which a certain parameter is changed. In this case, the only variant was the ocean’s initial state. The researchers sought to answer a riveting question: What were the chances that oil, exuding at a rate ranging from an estimated 1,000 barrels a day in April to upward of 62,000 barrels a day in August, would escape the Gulf and lurch up the Atlantic Seaboard and beyond to the coasts of Europe?
Meanwhile, other models were capturing world attention because they used real-time data and monitored the situation the way weather is forecast. Businesses, tourists, governments, environmentalists – all wanted to know the spill’s effect on local beaches, on ocean flora and fauna, and on seafood safety.
“We sought a different approach,” Maltrud says. “We didn’t try to make predictions, but tried instead to understand statistical distributions of what’s possible because that’s the kind of thing that our model can do. Those other models tried to represent exactly what was going on in the ocean at a given time, so we figured that we would try to shed some light on how long it might take oil to leave the Gulf, where it might go, and in what relative concentrations in relation to what was being released at the spill site.
The model Maltrud ran is called POP (Parallel Ocean Program). He made major contributions to its development at LANL in the ’90s; it is the ocean component of NCAR’s Community Climate System Model.
The crux of the modeling problem was a Gulf feature known as the Loop Current, a complex “clockwise surface circulation” entering the Gulf through the Yucatan Channel, and exiting in the Florida Current through the Florida Straits. Maltrud calls the Loop Current the “big meander” because its behavior is akin to a river on a flood plain: now and then changing course, creating oxbow lakes and mini-currents. In the Gulf, these small currents are eddies, subsets of the major loop circulation. The eddies are complex because there may be two or three going at any given time, each broken off from the Loop Current at its own interval and moving in its own pattern.
Their modeling arsenal included a powerful 120-year archive of oceanic dynamics data created over two-and-a-half years for an earlier study tracking chlorofluorocarbons (CFCs) in the ocean. CFCs are manmade chemicals used in refrigerants from the 1930s to the 1990s (see sidebar).
“The archive provided a robust bunch of different kinds of initial Loop Current conditions for the simulations, giving us a real idea of what the distribution of possible outcomes might be,” Maltrud says.
Time was the enemy. “We had never modeled oil before,” he says. “It really was a question of what could we credibly do in a really short amount of time. We were less concerned about the detail of oil than how the ocean dynamics drive the system, whether it’s the main current, the Big Loop, or the eddies, where the stuff goes and how long it takes to get where it’s going.”
Real oil, Peacock says, degrades over time as bacteria consume it, but the rate at which it can be broken down depends on multiple environmental variables and is highly uncertain. Also, crews started spreading large amounts of dispersants soon after the spill. The dispersant doesn’t actually reduce the amount of oil, but it does break it into tiny droplets that easily mix throughout the upper water column, effectively removing its signature from the surface ocean.
“We had no idea how to simulate dispersant use and didn’t want to try to simulate bacterial degradation because the rates of breakdown are so uncertain,” Peacock explains.
So she and Maltrud decided to model a “passive dye,” which would not degrade and break up, but would just be transported by the ocean currents. Besides, Maltrud already had coded the model to handle a surface dye at any specified location.
Says Peacock, “With dye we knew from the start what all the caveats were, and it took something like a day or two of talking about it to get the model running.”
The model calculates what’s happening at each point in a grid spread throughout the ocean area modeled. The grid boxes are spaced between 1 and 10 kilometers apart, depending on latitude. One side measures roughly 10 km at the equator, Peacock explains, but “they shrink as you go towards the poles” because the meridians converge. At high latitudes the same box is only 2 to 3 km on a side in the east-west direction but still measures roughly 10 km in the north-south direction.
The model is unique in that it accounts for eddies in its small grids. Climate models with grids spaced at 100 km or more must come up with ways to include ocean eddy mixing effects in the simulation parameters. The researchers first simulated continuously injected dye at the Deepwater Horizon drilling rig site for a two-month period, then ran simulations at four months out, all starting with different Loop Current configurations.
“The results gave a very quantifiable look at what the effect of the ocean state has on how long it takes the dye to leave the Gulf of Mexico,” Maltrud says.
Concludes Peacock, “The results from the multiple simulations suggested that it would be highly likely that dissolved oil would be transported through the Florida Straits roughly six months after the initial spill date but that no oil whatsoever would reach Europe.”
At year later – in April 2011 – the simulation shows dispersed oil as almost undetectable in the Atlantic Ocean.
“The concentrations in the model at one year post-spill are so low in the North Atlantic – about one ten-thousandth of the original concentration – that it’s basically undetectable,” Peacock says. “You would have to go out with really highly technical precision instruments to even sort the Deepwater oil from the baseline amount out there from tankers and lesser spills.”
“No one had ever done this sort of thing before,” Maltrud notes. “Granted, it’s a one-off sort of study for us, but I feel good about what we did, even though it was extremely idealized.” The study attempted “to show what the ocean can actually do under certain variables. We knew that this event was going to be measured really well and as the data will begin to be reported, we can use it to evaluate and improve our model. Eventually the whole experience will enhance our major thrust, which is climate and the interactions of the ocean with the atmosphere.”