A long view of Gulf oil spill

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.”