Why is oil spill hard to predict? Look to the clouds.
The BP Gulf oil spill has become the most recent example of a "Black Swan" -- i.e., an improbable, unpredicted event having tremendous impacts.
Retrospective analysis might demonstrate the Deepwater Horizon explosion and resulting massive flow of oil into the Gulf was a relatively likely "accident" waiting to happen. Without benefit of hindsight, though, it's been an event totally unanticipated with far-reaching consequences and one due to human failings, not an unavoidable natural disaster (e.g., earthquake, hurricane). Avoidable or not, we are in the position now of having to manage to the extent possible what heretofore was an unimaginable crisis.
The ability to manage the crisis is dependent upon at least three fundamental questions: Exactly how much oil has spilled? Where is the oil located? And where will it go? Unfortunately, there appears to be considerable uncertainty on answers to the first two questions, while the third is probably unanswerable beyond a few days in advance.
A reasonable analogy is predicting the development and motion of a cloud where imperfect measurements of its dimensions, location and background wind field are magnified over time by imperfect forecast models. To extend this analogy consider the following...
Oil: A column of oil gushing from the bottom of the Gulf reaches the surface where it spreads out more or less uniformly as a thin sheen over an increasingly large area.
Cloud: A plume of rising moist air reaches the level of condensation (beneath a stable layer of air that prevents it from rising further) where the newly formed cloud droplets spread out as a thin canopy over an increasingly large expanse of the sky.
Oil: Over time, wind, waves and local surface currents begin to break up the oil into an array of congealed lumps of varying size and thickness. These lumps tend to spread apart and organize non-uniformly into clusters and long streaks (streamers) as functions of the variability in the strength and direction of surface winds and currents, surface wave characteristics (height, period, etc.), degree of mixing with sea water, and vertical temperature and salinity structure of the underlying water column.
Cloud: Over time, the stable layer of air destabilizes (allowing the moist air to penetrate higher) with the original thin cloud being replaced by puffy cumulus clouds of varying horizontal and vertical dimensions. The individual clouds tend to organize into cloud clusters and lines of clouds spreading across the sky non-uniformly as functions of the variability in the strength and direction of winds, degree of mixing between cloud and surrounding air, and vertical stability of the air mass.
Oil: At least some oil will come ashore unexpectedly in the concentrated form of, for example, tar balls and mousse at the whim of winds, waves and currents. What might otherwise have been a great day for the beach or exploring the wetlands along the Gulf coast becomes a panicky rush to clean up and/or leave.
Cloud: Some cloud clusters unexpectedly develop into thunderstorms at the whim of local atmospheric conditions. What otherwise was a great day to be outdoors becomes a panicky rush to pack up and seek cover.
(I realize the analogy breaks down when considering that the long-term danger and after-effects of thunderstorms most certainly are not comparable to the long-term, massive, far-reaching impacts of the oil spill. But, please stay with me.)
Oil: Eventually there is a good chance the oil spill will encounter the Loop Current, which would carry it and related consequences around (or between) the Florida Keys and be picked up by the Gulf Stream.
Cloud: There's a good chance the further development and motion of the cloud clusters will be governed by the jet stream, which propagates them and associated weather far beyond the area of initial development.
Oil and cloud prediction
Predicting where the oil spill and its component parts will go is a challenge no less difficult, and probably even more difficult, than predicting clouds and overall weather conditions.
Oil on the surface is moved by both winds and currents. Specialized computer models, such as those used to help produce NOAA's oil trajectory maps, account for the influence of both by combining wind forecasts from weather models with current forecasts from ocean models. Not surprisingly, the predictability of oil spill movement is limited, since all weather and ocean models have errors that grow with time for a variety of reasons (e.g., initial condition uncertainties, imperfect models).
Forecasts of near-surface winds are generally not reliable beyond several days ahead (say, a week or so), as is true for all parameters within any weather forecast model, including those relevant to cloud development and motion. Even if winds were accurately predicted, wind-driven waves can disperse and spread the oil in directions that differ unpredictably from the wind. Moreover, observations are evidently lacking to precisely define the size and, especially, the small-scale structure of the oil spill. The latter is critical for predicting details of the location and timing of oil reaching vulnerable spots ashore -- not unlike difficulties in predicting the timing and location of thunderstorms.
While wind and waves play a critical role in details of oil movement, it's likely that large, strong currents that evolve relatively slowly over time, most notably the Loop Current and Gulf Stream, will ultimately govern how much of the oil is carried elsewhere from the Gulf. To the best of my knowledge ocean models, which suffer from an extreme scarcity of the observations necessary to define the initial ocean state and from imperfect representation of relevant dynamical and physical processes, have limited skill beyond a few days in predicting the evolution of such currents.
Next stops for oil?
The biggest threat in the shorter term is for the states of Louisiana, Mississippi and Alabama. The oil could also spread as a function of changes in wind speed and direction eastward toward the west coast of Florida and/or westward to the Texas coast. Eventually, it's not unlikely some oil will exit the Gulf of Mexico and move northward along the Atlantic coast and beyond. The least worst-case scenario would be for the oil to continue gushing into the Gulf for a prolonged period such that odds increase dramatically that all these areas would be affected. Even worse would be that the oil results in the Gulf of Mexico and much of the western Atlantic becoming dead zones for virtually all marine life.
Presumably everything possible will be done to avoid such worst-cast disasters, but failure in these efforts and resulting catastrophic consequences are not outside the realm of possibilities.
| June 10, 2010; 10:50 AM ET
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