When the Tohoku tsunami struck the northeast coast of Japan last March, retreating waves swept an estimated 5 million tons of debris out to sea. About 70 percent is believed to have sunk to the seafloor, leaving about 1.5 million tons floating, adrift somewhere on the Pacific Ocean.
Apart from a few large wind-driven objects tall enough to act as sails (such as the “ghost” fishing vessel scuttled last week off the coast of the U.S.), most of the debris is at or slightly below the surface and driven by ocean currents.
Standing tall enough above the water to be driven by winds, the 164-foot Japanese fishing vessel Ryou-Un Maru was spotted off the Coast of British Columbia in January. Determined to be a maritime hazard, the ship was scuttled by the U.S. Coast Guard last week, about 180 miles offshore in waters more than 6,000 ft. deep. The vast majority of tsunami debris, however, sits at or just below the surface. Driven by ocean currents rather than winds, it travels more slowly.
Initially, such surface debris (modeled in the animation above) was carried by the potent Kuroshio Current,that sweeps past eastern Japan much like the Gulf Stream in the North Atlantic. The Kuroshio carries surface waters northeast, before eventually turning east in the Kuroshio Extension and then the North Pacific Current.
Within a year or two, some surface debris is expected to reach the west coast of North America, but most is likely to end up in the North Pacific Gyre, commonly known as “the garbage patch.”
As of this month, there have been a few reports of debris at Midway Island and Kure Atoll. North winds have been minimal in recent months, and ocean currents have favored keeping the debris from the island. But those currents may be shifting, and debris should eventually wash up with greater frequency.
The animation above from NASA’s Earth Observatory was created using data from Surface Currents from Diagnostic (SCUD) model to simulate where and how surface debris would disperse.
Orange and red shaded areas represent waters with a high probably of debris. The deeper the red color, the higher the likely concentration. The debris field stretches roughly 5,000 kilometers by 2,000 kilometers across the North Pacific.
The SCUD model was developed by Nikolai Maximenko and Jan Hafner of the International Pacific Research Center (IPRC) at the University of Hawaii. Using real satellite data on sea surface height and on ocean surface winds—as well as information from scientific drifting buoy networks—the IPRC team makes daily maps of surface currents. Hafner and Maximenko have also collected reports of debris sightings in the ocean, which have so far validated what the model tells them.
