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Internal waves in the North Australian Basin and the Java
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Internal waves in the North Australian Basin and the Java Sea

November 11, 2010
On November 10, 2010 the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite passed over Indonesia, capturing this true-color image of internal waves emanating from the Lombok Strait and flowing into the North Australian Basin and the Java Sea. One set of waves, appearing as closely-spaced arcs, moves to the northeast into the Java Sea while a second set, with a broader and shallower pattern, appears in south.

The Lombok strait, a small sea channel between the islands of Bali and Lombok, is an important part of the Indonesian through-flow, an ocean current which transports relatively warm water from the Pacific Ocean to the Indian Ocean via the Indonesian archipelago. The floor of the Lombok strait contains a sill, a horizontal ridge formation, in waters with a depth of less than 350 meters, which affects the water flow. In the southern part of the strait a huge amount of water, sometimes more than 4 million cubic meters of water per second, is pushed over the sill, creating a large speed current. Both this large flow of water and the strong tides in the region work together to create unusual ocean phenomena, such as the generation of internal waves.

Internal waves occur because the ocean is layered. Deep water is cold, dense, and salty, while shallower water is warmer, lighter, and fresher. The differences in density and salinity cause the various layers of the ocean to behave like different fluids. When tides drag the ocean over a shallow barrier such as a ridge on the ocean floor, it creates waves in the lower, denser layer of water. These waves, internal waves, can be tens of kilometers long and can last several hours.

As internal waves move through the lower layer of the ocean, the lighter water above flows down the crests and sinks into the troughs. This motion compresses surface water in the troughs and stretches it over the crests, creating alternating lines of calm water at the crests and rough water at the troughs.

It is the pattern of calm and rough water that makes the internal wave visible in satellite images. Calm, smooth waters reflect more light directly back to the satellite, resulting in a bright, pale stripe along the length of the internal wave. The rough waters in the trough scatter light in all directions, forming a dark line.


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