A new computer model for the spread of marine debris from the March 2011 Japa­nese earthquake and tsunami shows that a large portion of the wind-driven flotsam has already arrived along the West Coast, while the main Hawaiian Islands and Northwestern Isles have so far been spared.

The new model better reflects the effects of wind on the lighter items, especially those that are exposed to air, said the scientists, Nikolai Maximenko and Jan Hafner, both with the International Pacific Research Center.

Last fall the scientists were concerned about debris hitting the sensitive Papahanaumokuakea Marine National Monument, but a lucky ocean quirk appeared and endured long enough to keep the debris to the north, said Maximenko, IPRC senior scientist.

At the end of September, the Russian training ship Pallada picked up verified tsunami debris about 400 miles northwest of Midway.

"In most other years, the currents would have continued to shove the debris toward the sanctuary," Maximenko said in a report Aug. 3 on the center’s website. "To our surprise, our model showed during October an oceanic front developing 200 miles northwest of Midway. The current associated with the front started to channel the tsunami debris northeast, north of the Hawaiian Islands. This front held and has protected the sanctuary."

Oceanic fronts are boundaries between water masses of different density, a function of temperature and salinity.

According to the model, the greatest concentration of wind-driven debris has passed north of Hawaii.

The original model developed by Maximenko and Hafner, a senior computer programmer, was more appropriate for heavy debris, mostly immersed and flowing with the currents. That should hit the U.S. and Canadian coast early next year, the scientists said.

An estimated 1.5 million tons of debris is floating eastward across the Pacific as a result of the March 11, 2011, quake and ensuing seismic sea waves.

During the first months after the disaster, all types of debris floated mostly along the same path because the dominant Kuro­shio Current is so fast that light summer winds played a minor role, the scientists said.

But the current typically weakens and winds grow stronger in the fall and winter, propelling the lighter, exposed debris faster than the heavy, mostly submerged junk.

By this summer a large portion of the wind-driven debris had already arrived at the West Coast, and the rest is in the central North Pacific, the scientists say.

An animation based on the model can be viewed at http://iprc.soest.hawaii.edu/news/marine_and_tsunami_debris/debris_news.php.