An internal wave propagating on the interface between two layers. The undisturbed sea level is indicated by the yellow line. Water particles are shown as yellow and magenta dots. Yellow dots sit in the middle of the water column and move only up and down. Magenta dots sit at the top and bottom of the water column and move only in the horizontal.
Watch the animation and concentrate on one of three aspects at a time:
By watching a yellow dot you can see how a water particle in the middle of the water column moves up and down, but does not move horizontally, as the wave passes through.
By selecting a particular magenta dot at the bottom of the water column and watching it you can see how a water particle moves back and forth horizontally as the wave passes. By comparing it with the movement of a particular dot above it you can see that at any one location, particles at the top and bottom of the water column move always in opposite direction.
By watching groups of magenta dots you can see that convergences (where water particles cluster together) and divergences (where particles are spread out) follow the wave, and that convergences are always located where the respective layer is thickest, while divergences are found where the layers are thinnest.