Internal Waves on the Sea of Japan
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Internal Waves on the Sea of Japan

June 18, 2009
This image, captured by the MODIS on the Terra satellite on June 11, 2009, shows an interesting phenomena called "internal waves". These particular waves are in the southern part of the Sea of Japan. South Korea is to the west, and Japan to the southeast.

Internal waves are waves that occur underwater at the boundary between layers of water with different densities. Like all major bodies of water, the Sea of Japan 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 body of water 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."

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