  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).
