Three decades ago researchers discovered what are essentially enormous saltwater lakes in the Atlantic Ocean. These “lakes,” called meddies, are gently spinning lenses of water up to 100 kilometers across and one kilometer thick. They float a few hundred meters below the surface of the ocean. Such large, warm bodies, which turned out to come from the Mediterranean Sea, should have an impact on heat exchange in the ocean—and on the planet’s climate. But efforts to study meddies—conventionally by dropping probes that directly measure the ocean’s temperature, salinity and velocity—have proved too costly, infrequent and spread out to reveal how the meddies dissipate their heat.
Now researchers have demonstrated that a tool adapted from the oil industry can take rapid, high-resolution snapshots of the meddies. The technique, first used to find oil deposits under the seafloor, exploits sound reflections. Prospectors on ships fire air guns just below the sea surface; the acoustic waves then propagate down through the seafloor and bounce back to a towed array of microphones. The timing of sound waves’ return reveals the density of the material through which they passed.
Boundaries between bodies of water also have a very faint sonic signature, which the oil industry used to treat as noise. But in 2003 a team led by W. Steven Holbrook of the University of Wyoming adopted the technique and created unexpectedly clear acoustic images of density boundaries in the ocean. Changes in the density of seawater are interpreted as changes in its temperature and salinity. Because these properties tend to be unique to each ocean current, the researchers could visualize interactions between ocean fronts, much like climatologists map the boundaries of weather fronts.
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Since then, researchers have analyzed old oil industry surveys and cobbled together experiments that could be piggybacked on oceanographic and oil industry cruises. Using data from a 1993 seismic survey off Spain’s southwestern coast, a team led by Valentí Sallarès of the Marine Technology Unit of the Spanish National Research Council in Barcelona reports in the June 14 Geophysical Research Letters that it has imaged three meddies in unprecedented detail.
Sallarès’s seismic images reveal “salt fingers” and other mixing features as small as 10 meters across. “At first blush, it’s just exciting for people to be able to see these things,” says Raymond Schmitt, an oceanographer at the Woods Hole Oceanographic Institution. But Schmitt says he and his colleagues are still grappling with how to interpret seismic images of meddies and other ocean-mixing hotspots such as underwater waves and the boundaries between ocean currents.
Seismic profiling is still not widely used in the oceanography community, in part because nobody has published a reliable quantitative conversion between seismic and traditional oceanographic measurements. Seismology detects reflections from places where the speed of sound changes. Oceanographic probes directly measure water conditions. Sallarès hopes to unify the two types of data: “The first step was the images, but if we’re not capable of quantifying mixing processes we won’t have anything.”
Sallarès says that preliminary results from a recent dedicated seismic oceanography cruise suggest that temperature and salinity values may be harder to distinguish than originally thought. Holbrook, who led his own seismic oceanography survey off the coast of Costa Rica in April, wrote from his research vessel that seismic oceanography needs to “produce exciting and useful quantitative results” so that oceanographers can view it as “a critical enhancement of their toolbox, rather than a curiosity.”
“I’m also hoping that we don’t exhaust the patience of the physical oceanography community while we develop the necessary techniques,” Holbrook adds. Researchers from both sides of the Atlantic will be gathering in November near Gerona, Spain, to share results from recent expeditions and to hash out the field’s next steps.
In the meantime, nobody knows exactly what meddies contribute to the Atlantic’s mixing, but Sallarès says that seismic profiling “is a clear first look and is more precise than what the oceanographic data can give us.”
Note: This story was originally published with the title, "Listening to a Mix".
