Internal Waves in the Atlantic Ocean, Northeast of Brazil. Northeast of Brazil, out in the deep waters of the Atlantic Ocean, sunglint highlights internal waves curving out toward the northeast. Internal waves are waves that occur underwater at the boundary between layers of water with different densities. Like all major bodies of water, the Atlantic Ocean is composed of layers of water with different densities: the topmost is the least dense, while each successively deeper layer is denser.
Internal waves are usually caused by the lower layer being forced against a shallow obstacle, such as a ridge, by tidal action. The ridge causes a disturbance, which creates a wave in the water layer, similar to the way that the wind can cause waves on the water´s surface. Unlike normal surface waves, internal waves can stretch for tens of kilometers in length and move throughout the ocean for several days.
Internal waves alter sea surface currents, changing the overall “sea surface roughness.” Where these currents converge, the sea surface is more turbulent, and therefore brighter because it catches more of the Sun´s reflection. Where the currents diverge, the surface is smoother and darker, creating zones called “slicks.”