February 11, 2013 - Atmospheric gravity waves off northwestern Australia
In late January, 2013 the calm waters of the Indian Ocean reflected the rolling waves of the atmosphere just above – and sometimes touching – its surface. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra satellite captured this true-color image of atmospheric gravity waves on January 30.
Long parallel arcs form a beautiful pattern off the shores of northwestern Australia. These are not rolling ocean waves. Instead, the air above the water has been disturbed as it blows off the coast, causing it undulate as it passes over the ocean’s surface. At the troughs, where the air falls, it touches the surface of the water. Because the touch occurs in an area of sunglint, the otherwise invisible interaction of atmosphere on ocean becomes beautifully visible.
When the Sun reflects off the surface of the ocean at the same angle that a satellite sensor is viewing the surface, a phenomenon called sunglint occurs. In the sunglint area of a MODIS image, smooth water becomes a silvery mirror, while rougher surface water appears dark.
Atmospheric gravity waves form when buoyancy pushes air up, and gravity pulls it back down. On its descent into the low-point of the wave (the trough), the air touches the surface of the ocean, roughening the water. The long, vertical dark lines show where the troughs of gravity waves have roughened the surface. The brighter regions show the crests of the atmospheric waves. Beneath the crests, the water is calm and reflects light directly back toward MODIS. Clouds commonly form at the crests of the waves. A band of clouds can be seen riding the western-most and the northern-most arcs.