Arrays of vortices in shallow fluids: Three-dimensional structure and dispersion

► We have studied the 3D structure of a linear array of EM-forced vortices in a shallow layer. ► Locally strong 3D circulations and rapid vertical mixing of particles despite the shallowness. ► The compressible horizontal surface flow strongly affects the spatial distribution of particles. ► 3D (weak) flow structure leads to dispersion of particles that is crucially different from 2D flows. ► The transport of passive tracers in a shallow fluid layer is basically of 3D nature.