A team from the Institut des sciences de la mer de Rimouski (ISMER), the Maurice-Lamontagne Institute (IML) and the Québec-Océan network has published in the prestigious scientific journal Nature Communications a major discovery on the origin of huge underwater waves.
From a series of sound scans carried out in the Saguenay Fjord (Canada), in a manner comparable to medical ultrasounds but with more powerful sounders embarked on research boats, the team has directly witnessed the birth of huge, 10-meter high, underwater waves accompanied with severe turbulence and large swirls.
After further data analysis and computer simulations, they have discovered that those underwater waves were generated by the collision of two opposing ocean currents having different temperature and salinity. Such collisions of ocean currents are known to be frequent in coastal seas, but it wasn't known that they could really cause such large underwater waves. This mechanism could therefore represent an important source of large underwater waves in the ocean, but possibly also in lakes and in the atmosphere where such waves are also known to exist.
Indeed, waves and breakers are not restricted to the surface of lakes or oceans. Although being counter-intuitive, the interior of the ocean is also filled with underwater waves. Those waves are not anecdotal. They can be found at any depth, from the first meters under the sea surface down to the bottom of the abyss. They are also much larger than surface waves, with heights that can reach several tens and even hundreds of meters depending on the location and oceanographic conditions.
Underwater waves can also break within the interior of the ocean in a manner comparable to wave breaking on beaches. When this happens it causes a lot of turbulence and provides a way for mixing the properties of the ocean such as temperature, salinity, nutrients, pollutants etc. In the absence of turbulence, those ocean properties tend to be organized into distinct layers throughout the depth of the ocean.
For example, saltier and colder waters are generally found lying underneath warmer and fresher waters, but the stirring caused by the breaking of underwater waves can significantly alter this distribution, and therefore ultimately affect the climate of the ocean.
Some underwater waves also have funky behaviors as they can travel not only horizontally, as do surface waves, but also vertically. So an underwater wave generated, say, near the surface could well propagate obliquely downward. Such a wave may eventually reach and bounce off the seafloor, thousands of meters down, to then propagate obliquely back up, bounce off the sea-surface, and propagate back down again, and so on.
Those underwater waves can propagate like that tens to hundreds of kilometers before eventually loosing their energy, possibly by breaking when encountering geographical obstacles such as continental slope or the flank of an island.
Observational evidences collected so far by oceanographers suggest that underwater waves are ubiquitous in the ocean and that they play a fundamental role in the functioning of the ocean, the marine ecosystem and the climate. However, the physics of underwater waves remains poorly understood and is a hot research topic in physical oceanography.
While the wind is virtually the only source of generation of surface waves, physicists do not yet fully understand what actually generates underwater waves. Various mechanisms, based on theories or on experiments carried out in laboratory tanks, have been proposed to explain the existence of those waves, but few of those mechanisms have actually been clearly observed at sea.
Yet, the very nature of Science is to confront theories and models to the observations of natural phenomena. This discovery represents a rare case of the generation of huge underwater waves directly observed at sea and for which the generation mechanism has been elucidated.
Reference and free access to the paper: Bourgault D, Galbraith P S and Chavanne C 2016 Generation of internal solitary waves by frontally forced intrusions in geophysical flows, Nature Communications, 10.1038/NCOMMS13606
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