There have been numerous instances wherein both scientists and researchers could have predicted certain aftermath impacts of natural disasters. Continuing with our previous posts we look into some more interesting modern ways which could help us cope and even predict natural disasters in the future.
The most dramatic demonstration of the transport barrier concept came in the aftermath of the 2010 Gulf oil spill. Oceanographers and mathematicians have analyzed the huge volumes of data on the spill and shown how the information could have enabled scientists to better predict where the oil would go. Lagrangian coherent structures might help explain why the surface oil disappeared more rapidly than anyone expected— much faster, for example than the oil from the Exxon Valdez spill in 1989 in Prince William Sound in Alaska.
The fate of the subsurface oil has been more controversial, and much of it may still remain at the bottom of the Gulf. The warm Gulf of Mexico, it turned out, is home to hordes of microorganisms that feed on hydrocarbons that naturally seep into the Gulf waters. Given a much more abundant supply of hydrocarbons than usual, these microorganisms flourished. Microbiologist Dave Valentine and mathematician Igor Mezic, both at the University of California, Santa Barbara, showed that the bacteria tended to congregate in coherent regions defined by transport barriers.
Clearly, the long-term stability of those regions helped the oil degrade. Valentine notes that it would have been a different story if the blowout had happened off the coast of Brazil, another region where vast deepwater oil reserves have been discovered. There the currents lead out to sea, where a captive supply of bacteria does not exist to chow down on the hydrocarbons.
Transport barriers may also explain why the oil from Deepwater Horizon avoided flowing into the Loop Current, a persistent jet that leads through the Florida Straits and into the Atlantic, where it could have polluted beaches along the East Coast. As late as July 2, the National Oceanic and Atmospheric Administration was predicting a 61 to 80 percent chance some oil would make it to the Loop Current. The prediction was based on 15 years of historical ocean current data from the Gulf of Mexico. In 2010 we apparently got lucky.
First, unusually strong winds from the Southwest pushed the oil slick to the north, away from the Loop Current. In addition, a giant eddy, called Eddy Franklin, detached from the Loop Current and pushed it farther south than usual, forming a barrier between the oil and the current. It remains to be seen whether any of these phenomena could have been anticipated. Haller, however, with oceanographer Maria Olascoaga of the University of Miami, has shown that other seemingly capricious changes in the oil slick were predictable. On May 17, for instance, a giant “tiger tail” (named after its shape) of oil suddenly traveled more than 160 kilometers southeast in one day. According to their computer analysis, the tiger tail traveled along an attracting Lagrangian coherent structure, and the impending instability was presaged seven days earlier by the formation of a strong attracting “core” on that structure.
Likewise, an abrupt westward retreat of the oil slick’s leading edge on June 16 was anticipated nine days earlier by the formation of an exceptionally strong repelling core to the east of the slick. Had surveillance been in place that could identify transport barriers, cleanup boats could have been sent to the right locations.
Beyond the study of oceanic currents, applications of the transport barrier concept have proliferated in recent years. For example, Shane Ross of Virginia Polytechnic Institute has studied the effect of transport barriers in the atmosphere on airborne pathogens. He and plant biologist David Schmale, also at Virginia Tech, used a small drone airplane to collect air samples at an altitude between tens and hundreds of meters above Blacksburg. When an attracting structure passed by or when two repelling structures passed in rapid succession, the researchers detected a spike in the number of spores of a fungus called Fusarium.
Ross hypothesizes that in the first case the spores had been pulled toward the coherent structures, whereas in the second they had become trapped between the two repelling barriers, like cattle herded into a small region by prods. Some of the spores were of a species that does not usually occur in Virginia, which suggests that the structures remained intact long enough for the spores to be transported several hundred kilometers. Shadden is now studying the role of Lagrangian coherent structures in blood flow. For example, he has used these structures to reveal the boundaries between blood ejected on one heartbeat and blood ejected on the next. He showed that most of the blood in a normal ventricle remains there for at most two heartbeats. But in six patients with enlarged hearts, regions of blood recirculated for much longer—“a widely recognized risk factor for thrombosis,” he wrote in a draft of his study.
More than a decade after Haller named them, Lagrangian coherent structures are still far from being a mainstream tool in oceanography or atmospheric science. One objection raised about their usefulness is that if there are errors in the measurement of the flow field, they will surely propagate and produce errors in the predictions of the transport barrier as well. But the Monterey Bay experiment found that the location of the transport barriers was relatively insensitive to measurement errors. Another objection is that to compute the structures, you need to know the entire flow field, meaning the velocity of water flowing at each point. But if you know that, you can forecast the oil slick using existing computer models. So what are calculations of Lagrangian coherent structures good for?
As it turns out, forecasting is not the only game in town. “Hindcasting” may turn out to be important in finding the source of “mystery oil spills” that wash ashore from unknown sources—often from sunken ships. For example, the SS Jacob Luckenbach, which sank off San Francisco in 1953, polluted the California coast every year beginning around 1991, but the source of the spill was not discovered until 2002.
Plane crashes and shipwrecks have also produced “debris spills” and “body spills.” Because conventional ocean models cannot be reversed in time, rescuers cannot extrapolate backward from the observed debris field to find the source. Oceanographer C. J. Beegle-Krause and mathematician Thomas Peacock of the Massachusetts Institute of Technology are now working on using La grangian coherent structures to forecast where shipwreck survivors will drift in the currents, which would help narrow down the search area. In such situations, as Peacock notes, “even a few minutes might be a matter of life and death.”
Finally, Lagrangian coherent structures provide more than mere forecasts or hindcasts; they provide understanding. Knowing about the structures enables scientists to better interpret the predictions of computer models. If a model predicts that a filament of oil will move toward Pensacola and we can see a structure pushing it or pulling it that way, we can be reasonably confident in the prediction.
If there is no corresponding structure, we might treat the model with more skepticism. Mathematicians are now broadening their research into different types of organized structures in turbulent fluids, such as eddies and jets. With deeper understanding, we may be able to answer questions about chaotic phenomena that now elude us.
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