When two currents collide, they create eddies, like these cloud formations photographed from a satellite above the Canary Islands in the North Atlantic.
Eddies are also seen where the Oyashio Current collides with the Kuroshio Current near Hokkaido.
They show up because of phytoplankton growing in the surface waters. These become concentrated along the boundaries of the eddies, tracing out the motions of the water.
Eddies can also be induced (red I) on a conductive metal plate (C) as it moves under a magnet (N). The magnetic field (green B) is directed down through the plate.