CAMBRIDGE, MA, Jan. 9, 2014 -- Origins of underwater waves in the South China Sea towering hundreds of feet have been revealed in new large-scale tests conducted both in the lab and throughout the sea.
The waves' effect on the ocean's surface is minimal, producing a rise of just inches. However, internal waves, which are hidden entirely within the ocean, can tower hundreds of feet, with profound effects on the Earth's climate and ocean ecosystems. The new findings come from a team effort involving the Massachusetts Institute of Technology (MIT) and several other institutions, and coordinated by the Office of Naval Research (ONR).
These waves resemble surface waves in shape, but the only difference between them and the surrounding water is their density, due to temperature or salinity differences that cause ocean water to become stratified. Further, the boundary between colder, saltier water below and warmer, less-salty water above can be detected instrumentally. That layer can resemble the ocean's surface, producing waves that reach towering heights and traveling vast distances, as well as play a key role in the mixing of ocean waters, helping drive warm surface waters downward and drawing heat from the atmosphere.
Because these internal waves are hard to detect, it is often a challenge to study them directly in the ocean. But now Thomas Peacock, an associate professor of mechanical engineering at MIT, has teamed with researchers from the Ecole Centrale de Lyon, the Ecole Normale Superieure de Lyon, and the University of Grenoble Alpes, all in France, as well as the Woods Hole Oceanographic Institution, to conduct the largest laboratory experiment ever to study such waves. Their results have been published in the journal Geophysical Research Letters.
The team performed laboratory experiments to study the production of internal waves in the Luzon Strait, between Taiwan and the Philippines. "These are the most powerful internal waves discovered thus far in the ocean," Peacock said. "These are skyscraper-scale waves." These solitary waves have been observed to reach heights of 170 meters (more than 550 feet) and can travel at a leisurely pace of a few centimeters per second.
The team's large-scale laboratory experiments of the generation of such waves used a detailed topographic model of the Luzon Strait's seafloor, mounted in a 50-foot-diameter rotating tank in Grenoble, France -- the largest such facility in the world. The experiments showed that these waves are generated by the entire ridge system on that area of seafloor and not a localized hotspot within the ridge.
"It's an important missing piece of the puzzle in climate modeling," Peacock said. "Right now, global climate models are not able to capture these processes, but it is clearly important to do so." To help incorporate the new findings into these models, the researchers will meet in January with a climate-modeling team as part of an effort sponsored by the National Science Foundation to improve climate modeling.
These waves are potentially "the key mechanism for transferring heat from the upper ocean to the depths," Peacock said, so the focus of the research was to determine exactly how the largest of these waves, as revealed through satellite imagery of the Luzon Strait region, are generated. Among the new techniques that have helped to propel the field forward is the use of satellite data. While the submerged waves raise the surface of the water by less than an inch, long-term satellite data can clearly discern this difference.
Many locations, such as the Luzon Strait, generate these waves in a steady, predictable way as tides flow over submerged ridges and through narrow channels. A resulting 12-hour periodicity is clearly visible in satellite data. Beyond their effects on climate, internal waves can play a significant role in sustaining coral-reef ecosystems, which are considered vulnerable to climate change and to other environmental effects. Further, internal waves can bring nutrients up from ocean depths, Peacock said.