Except for two gaps, land hems in the Gulf of Mexico. As a result, ocean current tends to flow in through a gap between the Yucatan and Cuba, and then into a forceful clockwise Loop Current that turns toward the southeast.
Most of the time, the Gulf's Loop Current is able to squeeze out through the Florida Straits. "But it's unstable," says Jolliff, "so with some frequency, this big loop detaches and forms this sort of closed, clockwise circulation of waterwhat oceanographers call a mesoscale eddy." And with the hindcasts, Jolliff observed exactly that.
"You see that the Loop Current itself pinched off to form an eddy." This fortuitous event prevented the current from carrying aggregated surface oil to Key West and eastern Florida beaches, like Miami. "This emphasizes the point that the only way you'd know that is to forecast the ocean circulation."
In addition to understanding regional currents, time scale is also very important. For the 96-hour forecast, Jolliff could assume oil was an inert tracer. But over time, oil "weathers"evaporating and biodegrading. Jolliff accounted for weathering in the mesoscale by assuming a rate of decay. He acknowledges this oversimplifies the true chemical properties of oil, and could be improved in a future model; but in 2010, most forecasters didn't account for weathering at all.
Even if the current had carried oil to Florida, the damage likely wouldn't have been severe. "What we see is that it takes more than 10 days for that surface oil to finally get entrained into the Loop Current. Enough time has elapsed that these materials are now significantly degraded."
NRL on the front line: improving ocean models
|Contact: Kyra Wiens|
Naval Research Laboratory