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Abstract

We used transition path theory (TPT) to infer “reactive” pathways of floating marine debris trajectories. The TPT analysis was applied on a pollution-aware time-homogeneous Markov chain model constructed from trajectories produced by satellite-tracked undrogued buoys from the National Oceanic and Atmospheric Administration's Global Drifter Program. The latter involved coping with the openness of the system in physical space, which further required an adaptation of the standard TPT setting. Directly connecting pollution sources along coastlines with garbage patches of varied strengths, the unveiled reactive pollution routes represent alternative targets for ocean cleanup efforts. Among our specific findings we highlight: constraining a highly probable pollution source for the Great Pacific garbage patch; characterizing the weakness of the Indian Ocean gyre as a trap for plastic waste; and unveiling a tendency of the subtropical gyres to export garbage toward the coastlines rather than to other gyres in the event of anomalously intense winds. Given a Markov chain, namely, a model describing the stochastic state transitions in which the transition probability of each state depends only on the state attained in the previous event, transition path theory (TPT) provides a rigorous approach to study the statistics of transitions from a set of states to another, possibly disconnected set of states. Envisioning the motion of floating debris as described by a Markov chain that accounts for the ability of coastal states to “pollute the oceans,” TPT is employed to unveil “reactive” pathways representing direct transitions from potential release locations along the shorelines to accumulation sites across the world ocean. These include the subtropical gyres, whose strength in this context is investigated