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In March, a team of scientists dragged a blast furnace on a sled across a giant slab of ice in the Beaufort Sea, above the Arctic Circle. With the furnace, the researchers (from the United States Navy and the Massachusetts Institute of Technology) melted a hole in the ice big enough to fit their 850-pound, 12-foot drone, which they dropped through to the icy waters below. Their mission: to measure how climate change is altering the acoustics of the Arctic Ocean.
Understanding how sound travels under water is critical to the Navy because its submarines use sonar to communicate, and to track and identify foreign vessels. The speed of sound is variable in water, changing with temperature, salinity, and pressure. As temperature decreases, sound waves slow down; as pressure (or depth) increases, they speed up. Sound waves also tend to refract, or bend, toward water layers where the speed of sound is lower. As ice melts in the Alaskan Arctic—a strategic region that is increasingly heavily trafficked—and ocean temperatures shift, the temperature and position of ocean water layers shifts, too.
The Beaufort Sea used to have a very different thermal profile: frigid water flowed just below the surface ice, and it generally grew colder with depth. But since 1975, the water just beneath the ice has warmed almost 1.5 degrees Celsius due to an influx of warm water from Alaskan rivers and the Bering Sea. At the same time, a deeper layer has warmed a few tenths of a degree due to the influx of warm summer water from the Atlantic Ocean. These temperature changes have contributed to a phenomenon known as the Beaufort Lens, which acts as a sonic superhighway stretching between Alaska and the Northwest Territories. The Beaufort Lens consists of three layers: a layer of mixed warm and cold water near the ocean’s surface, a second layer of colder water at between 100 and 200 meters of depth, and 200 or so more meters beneath those, a layer of warmer water.
“We’re still trying to wrap our heads around exactly how big it is or how dynamic it is.”
Though scientists at Woods Hole have been monitoring the Beaufort Lens since 2004, it was only two years ago that Lee Freitag, a Woods Hole Oceanographic Institute acoustic engineer, first catalogued its sonic properties. Freitag learned that its distinct layer structure allows sound to travel underwater uninterrupted over more than 400 kilometers, about four times what would normally be expected.
Before the formation of the Lens, sound waves would bend toward the frigid waters just beneath the ice cover, whose rough surface would scatter them. Today, sound waves near the ice surface to refract downward toward the Lens’ colder, middle layer. Similarly, deeper sound waves refract upward to this cold layer. Once inside that cold layer, the waves effectively get trapped like a bowling ball stuck in a gutter, and so can travel long distances. Temperate and tropical oceans around the globe have similar acoustic channels known as deep sound channels.
The Navy hosts research camps on the Arctic ice every two years, and MIT research team leader Henrik Schmidt is hoping to break new ground on the Beaufort Lens in 2018 with a more precise and complex system for drone tracking and control. Says Scott Carper, a master’s student in the MIT lab, “We’re still trying to wrap our heads around exactly how big it is or how dynamic it is.”
Anna Nowogrodzki is a sciencejournalist in Boston who has written for Nature, National Geographic, New Scientist, Undark, Smithsonian, mental_floss, and MIT Technology Review. Follow her on Twitter @AnnaNowo.
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