N°42 : Langmuir's Circulations

You may already have noticed these lighter lines on the surface of the lake, especially on windy days. Known as the Langmuir Circulation, these bands are due to a phenomenon that takes place underwater in 3 stages.

1. The wind will generate, in addition to the waves, small sub-lake eddies sinking vertically. The eddies rotate alternately clockwise and counter-clockwise.
2. Under the impulse of the waves and the surface current caused by the wind, the whirlpools will gradually tilt until they are completely horizontal. Each roller is in line with the wind.
3. As the rollers alternate clockwise and counter-clockwise, zones of convergence and divergence are created in the spaces between two rollers. The zones of convergence will bring together everything that floats (foam, vegetation and other debris), while the zones of divergence will allow mixing for aquatic micro-organisms.

It was Irving Langmuir, an American physicist, chemist and engineer, who noticed this phenomenon after observing regular rows of plants and algae in the Sargasso Sea in 1927. Various studies have been carried out on this helical flow and its importance in the oceans. They show that a high concentration of organisms circulates in these convergence zones. This allows a mixing of nutrients and has a direct effect on the distribution of aquatic organisms such as plankton in the different layers of water. In this way, surface thermal stratifications are regularly mixed and oxygenated. Although relatively little is known and studied today, this mixing role is crucial for surface waters.
If you see this phenomenon on your next outing by the lake, or if you've already seen it, don't hesitate to share your experience and any photos you may have!

Information taken from articles :
Maurel Philippe (1997), Analysis and modelling of currents and turbulence under wind wavesPhD Thesis, INP Toulouse, 
David Calkins, The Basis of Langmuir Circulation,
Wikipedia, Langmuir Circulation,

Photo credit: ©David Calkins and Maurel Philippe

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