The Lagrangian motions of floating matter on the sea surface were simulated by using the surface current data based on shipdrift data produced by Meehl. The validity of the simulation was confirmed by comparing the results of the model with the trajectories of satellite tracked drift in the eastern North Pacific observed by Kirwan et al. Some cases which originated in the western North Pacific Ocean were investigated. It was found that drifters set in the ocean during spring quickly migrated to North America on the strong eastward North Pacific currents of the summer season. Trajectories started during autumn showed a loop in the western North Pacific and took more time to arrive in the eastern area of the North Pacific Ocean. Each trajectory that arrived in the eastern area of the North Pacific Ocean, showing a large loop, traveled over a one year interval owing to the large surface current vortex. This vortical sea surface current was driven by the clockwise winds around the atmospheric subtropical high pressure region located in the North Pacific.
Numerous calculations with initial positions randomly scattered in space and time were performed, and the accumulated matter density was obtained. High density areas where the debris concentrated were found at several places (i.e., near Berinuda, west of Australia, the center of the South Atlantic, and north of the Hawaiian Wands). In focussing on the North Pacific, it was found that three identifiable high density areas circulate with three year periods. It is emphasized that the instantaneous strong convergence areas do not always agree with the high accumulation density areas owning to the hysteresis or memory of the floating debris.