Inertia-induced accumulation of flotsam in the subtropical gyres

TL;DR

This paper reveals that inertia and buoyancy effects, rather than Ekman convergence, primarily cause the accumulation of marine debris in subtropical gyres, supported by a new Maxey--Riley model.

Contribution

It introduces a novel inertial model using the Maxey--Riley equation to explain flotsam accumulation in gyres, challenging previous Ekman convergence explanations.

Findings

Undrogued drifters accumulate in gyres similar to plastic debris.

Inertial effects cause faster accumulation than Ekman convergence predicts.

A new Maxey--Riley based model explains flotsam dynamics.

Abstract

Recent surveys of marine plastic debris density have revealed high levels in the center of the subtropical gyres. Earlier studies have argued that the formation of great garbage patches is due to Ekman convergence in such regions. In this work we report a tendency so far overlooked of drogued and undrogued drifters to accumulate distinctly over the subtropical gyres, with undrogued drifters accumulating in the same areas where plastic debris accumulate. We show that the observed accumulation is too fast for Ekman convergence to explain it. We demonstrate that the accumulation is controlled by finite-size and buoyancy (i.e., inertial) effects on undrogued drifter motion subjected to ocean current and wind drags. We infer that the motion of flotsam in general is constrained by similar effects. This is done by using a newly proposed Maxey--Riley equation which models the submerged (surfaced) drifter portion as a sphere of the fractional volume that is submerged (surfaced).