Numerical estimation of the hotspot positions of floating plastic debris in the Tsushima Strait using the adjoint marginal sensitivity method

Marine plastic has become a global problem and the situation is also very serious in the Sea of Japan. Tsushima Island, Japan is one of the areas encountering severe problems with beach plastics, followed by many other beaches facing the Sea of Japan. The total cost of beach cleanup is nontrivial for municipalities facing the sea. Offshore recovery of floating large debris at its concentrated areas, or hotspots, could be less burdensome and more economical than the beach cleanup. For this purpose, it is necessary to confirm the presence of hotspots in a target sea, and identifying offshore hotspots using a suitable numerical method can aid in the recovery of large floating plastics. In this study, we estimated the hotspot position of floating plastic bags in the Tsushima Strait using the adjoint marginal sensitivity method and measurement data in the Sea of Japan. The numerical results were found to be moderately comparable with satellite images using indices of large floating plastics.