Clustering of Marine‐Debris‐ and Sargassum‐Like Drifters Explained by Inertial Particle Dynamics

Drifters designed to mimic floating marine debris and small patches of pelagic Sargassum were satellite tracked in four regions across the North Atlantic. Though subjected to the same initial conditions at each site, the tracks of different drifters quickly diverged after deployment. We explain the clustering of drifter types using a recent Maxey‐Riley theory for surface ocean inertial particle dynamics applied on multidata‐based mesoscale ocean currents and winds from reanalysis. Simulated trajectories of objects at the air‐sea interface are significantly improved when represented as inertial (accounting for buoyancy and size), rather than as perfectly Lagrangian (fluid following) particles. Separation distances between simulated and observed trajectories were substantially smaller for debris‐like drifters than for Sargassum‐like drifters, suggesting that additional consideration of its physical properties relative to fluid velocities may be useful. Our findings can be applied to model variability in movements and distribution of diverse objects floating at the ocean surface. Plain Language Summary Predicting the fate of floating matter requires one to recognize that they respond differently than Lagrangian (i.e., infinitesimally small, neutrally buoyant) particles to the action of surface currents and winds. Indeed, the Maxey‐Riley equation of fluid mechanics—a Newton second‐law‐type ordinary differential equation—shows that even the motion of seemingly small neutrally buoyant particles immersed in a fluid in motion can substantively deviate from that of Lagrangian particles. The Maxey‐Riley equation has been recently extended to account for the combined effects of ocean current and wind drag on finite‐size particles floating at the ocean surface. We show here that the paths of drifters that mimic marine debris and small Sargassum patches cluster according to their inertial characteristics consistent with the Maxey‐Riley theory. Key Points Custom‐made marine‐debris‐ and Sargassum‐like undrogued surface drifters are observed to cluster according to design type The clustering is explained as a result of inertial effects using a novel Maxey‐Riley theory The results have implications for maintaining ecosystem health, search‐and‐rescue operations, and marine safety