File:Oceanic nonlinear internal waves in the Banda Sea (afternoon overpass) (MODIS 2015-11-09).jpg

English: Ocean waves and shimmering sunlight created a stunning interplay in the Banda Sea in late October 2015. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA�s Aqua satellite was flying over the Banda Sea on October 29, 2015, when it captured this true-color image of the phenomenon.

Internal waves are waves that travel within the interior of a fluid, rather than at water�s surface and they occur in stratified layers within the ocean. Although we might think often of water as one uniform body, that is hardly the case in the oceans. Temperature, salinity and other phenomena can create stratified layers of water of different compositions and densities. Like many areas, the Banda Sea is composed of water of differing densities, with the least dense water at the surface and each successive layer increasing in density. At times, a deep layer is disturbed and a turbulent wave is propagated in that layer � while upper layers move only slightly or not at all. Such internal waves are often caused by the lower layer being pushed against an obstacle, like a ridge on the sea floor, often by tidal motion. Unlike surface waves, which tend to dissipate relatively quickly, internal waves can move throughout the ocean over several days and may stretch tens of kilometers in length.

Although internal waves do not break on the surface, the moving water layer does affect the roughness of the surface. Where the internal waves converge, the sea surface is rougher and where the currents diverge the surface is smoother. These subtle changes in surface roughness are difficult to see in normal true-color images, but sunglint in an image can make otherwise invisible phenomena visible. When the sun is reflected nearly back to the sensor, it creates �sunglint� which appears as a silvery-gray coloration in the image. In areas of sunglint, rougher water appears a deep pewter and in calm area the water appears bright silver. Outside of the swath of sunglint the ocean water appears deep blue. This surface roughness combined with sunglint allows the visualization of otherwise difficult-to-see ocean internal waves.