What happened to the oil?

David Hosansky | 28 February 2011 • During last year’s crisis involving the massive release of oil into the Gulf of Mexico, NCAR issued a much-watched animation projecting that the oil could reach the Atlantic Ocean. But detectable amounts of oil never made it to the Atlantic, at least not in an easily visible form on the ocean surface. Not surprisingly, we’ve heard from a few people asking whether NCAR got it wrong.

 These events serve as a healthy reminder of a couple of things:

  • the difference between a projection and an actual forecast
  • the challenges of making short-term projections of natural processes that can act chaotically, such as ocean currents
loop current
The Loop Current takes various tracks, sometimes moving fairly directly from the Yucatan Peninsula to the Florida Straits and sometimes forming a large loop that extends into the northern Gulf. It stayed on an unusually persistent southerly track following the release of oil into the Gulf, which prevented it from reaching eddies near the spill site and transporting oil into the Atlantic. (UCAR illustration; NASA base map.)

First, the projection. Scientists from NCAR, the Department of Energy’s Los Alamos National Laboratory, and IFM-GEOMAR in Germany did not make a forecast of where the oil would go. Instead, they issued a projection. While there’s not always a clear distinction between the two, forecasts generally look only days or hours into the future and are built mostly on known elements (such as the current amount of humidity in the atmosphere). Projections tend to look further into the future and deal with a higher number of uncertainties (such as the rate at which oil degrades in open waters and the often chaotic movements of ocean currents).

Aware of the uncertainties, the scientific team projected the likely path of the spill with a computer model of a liquid dye. They used dye rather than actual oil, which undergoes bacterial breakdown, because a reliable method to simulate that breakdown was not available. As it turned out, the oil in the Gulf broke down quickly due to exceptionally strong bacterial action and, to some extent, the use of chemical dispersants.

Second, the challenges of short-term behavior. The Gulf's Loop Current acts as a conveyor belt, moving from the Yucatan through the Florida Straits into the Atlantic. Usually, the current curves northward near the Louisiana and Mississippi coasts—a configuration that would have put it on track to pick up the oil and transport it into open ocean. However, the current’s short-term movements over a few weeks or even months are chaotic and impossible to predict. Sometimes small eddies, or mini-currents, peel off, shifting the position and strength of the main current.

To determine the threat to the Atlantic, the research team studied averages of the Loop Current’s past behavior in order to simulate its likely course after the spill and ran several dozen computer simulations under various scenarios. Fortunately for the East Coast, the Loop Current did not behave in its usual fashion but instead remained farther south than usual, which kept it far from the Louisiana and Mississippi coast during the crucial few months before the oil degraded and/or was dispersed with chemical treatments.

The Loop Current typically goes into a southern configuration about every 6 to 19 months, although it rarely remains there for very long. NCAR scientist Synte Peacock, who worked on the projection, explains that part of the reason the current is unpredictable is “no two cycles of the Loop Current are ever exactly the same." She adds that the cycles are influenced by such variables as how large the eddy is, where the current detaches and moves south, and how long it takes for the current to reform.

Computer models can simulate the currents realistically, she adds. But they cannot predict when the currents will change over to a new cycle.

The scientists were careful to explain that their simulations were a suite of possible trajectories demonstrating what was likely to happen, but not a definitive forecast of what would happen. They reiterated that point in a peer-reviewed study on the simulations that appeared last August in Environmental Research Letters. These caveats, however, got lost in much of the resulting media coverage.

The long and the short of it

The research shows why it’s so difficult to make short-term projections of natural systems. Much of nature—including ocean currents, atmospheric winds, and the actions of living creatures—is hard to predict from one day to the next. Over time, however, certain patterns emerge.

Even trying to predict tomorrow’s weather is highly complex because the atmosphere contains so many variables, each interacting with each other in ways that are extraordinarily challenging to piece together.

Predicting longer-term trends is actually more straightforward in some respects because nature follows certain patterns over time. This is why, if you’re in the Northern Hemisphere, you can safely pull out your summer clothes in a few months: July is reliably warmer than January in most areas.

To look at it through the lens of personal experience: I was hiking with my wife a few years ago in Alaska when we came across a black bear. These bears are rarely aggressive (unlike grizzlies and polar bears). Still, black bear attacks have been known to happen, and to suddenly come across such a large animal in the woods is disconcerting. Fortunately for us, this particular bear followed the law of averages. It ambled away.

The currents in the Gulf last summer, on the other hand, did not exhibit average behavior—which, as it turned out, was a good thing for the Atlantic Coast. Although the spill created an ecological disaster in the Gulf, the damage could have been far more widespread. It’s a case where we can be glad that natural processes did not behave as expected.