Langmuir Circulation



19/10/2017

Piu Mandal*

Langmuir Circulation is a kind of water circulation mainly driven by wind. It is the most important topic in Physical Geography and also most important research topic to the Oceanographer. The study of Langmuir Circulation is quite interdisciplinary, as it is a cross idea from Physics, Chemistry, Biology and Applied Mathematics. Langmuir Circulation process or mixing the ocean layer mainly incorporated with fluxes of momentum, heat, mixing gases across the air-sea interface and transport of nutrients and others. There lies a crucial link in coupling the air and sea for mixing layer in ocean surface.
What is Langmuir Circulation?
“Langmuir Circulation is shallow, slow, counter-rotating vortices at the ocean's surface aligned with the wind”. These are mainly found when wind blows over the sea surface. This type of circulation is mainly seen when convergence of two nearby cells form a parallel pattern on the water surface. This convergence zone or area is popularly known as ‘Wind Streak’ or ‘Windrows’ in the field of Oceanography. Langmuir Circulation mainly developed when wind blows on the sea surface as a steady format. But Circulation mechanisms are not easy to understand, as many researcher trying to find out the complex interaction between air and wave and currents of the ocean.
Formation of this type of ocean circulation, create a cell orientation of water flow or movements called as Langmuir Cell. These types of cells are formed at topmost part or at 6 meter of the water column. ‘Windrows’ or ‘Wind Streak’ is the interaction zone of the adjacent cells  in opposite direction, causing upwelling and down welling of water volume. Convergence zone or Currents identified as presence of streaks on the sea surface.

Discovery
In 1936, Irving Langmuir, an eminent scientist first discover this type of circulation during his cross-Atlantic Voyage. This type of thought had come to his mind when he saw floating Sea weeds (Saragassum) in long, parallel linear pattern on the water surface. Later he conducted different experiments in Lake George, New York to explain this type of water formation. Langmuir found that the streaks which oriented approximatly parallel to the wind, are line of surface convergence beneath which there exists downward motion while between streaks there is upwelling. These water motions have subsequently been called Langmuir circulations.” (Sutcliffe, Baylor, and Menzel 1963).
Mechanisms and Process
Langmuir cell can be seen in oceans, seas, lakes, and even in rivers. The rotation of water primarily known as interaction of winds, waves, and sea currents but the exact formation is still not well defined. After a long research on this topic ‘Department of Earth and Atmospheric Science’, University of New York, mentioned six hypotheses to explain Langmuir circulation and their associate formation of wind streaks. These six hypotheses are: 1) Shearing Instability, 2) Atmospheric Vortices, 3) Wind-oriented thermal convection, 4) Wind profile modification by a surface film, 5) Radiation pressure on surface film, and 6) Convergence of two wave-trains.

In shortly, primarily wind takes main role behind this type of mechanism. As mentioned before adjoining cells are created and rotate in opposite direction, ‘Windrows’ or ‘Wind Streak’ mainly associated with convergent water or down welling mechanism and divergent water associated with upwelling mechanisms. In Fig.2: water circulation marked with deep blue area of adjoining cells.
Observational characteristics of Langmuir Circulation:
·        This type of circulation fairly observed in northern Hemisphere in between 0°-20°N latitude.
·        At the convergent zone, floating seaweeds, foam, flowing sediment and debris are concentrated.
·        This type of circulation, mainly imprint of convection cells.
·      According to an article in the journal Fluid Dynamics, Langmuir cells can be generated several meters too many Kilometres in length, but depth does not excessed six meter.
·        Wind speeds ranging from 2-12m/s maximum in this type of water circulation.
·        This type of water circulation can formed quickly and can last from several minutes to several days.
Lastly, Langmuir Circulation is a convective water circulation, fairly formed in ocean, lake, and also in river. This wind driven water motion mainly found in upper part of ocean. As a result temperature and density of water are relatively uniform in upper stratus of the ocean. This type of circulation pattern also helps to predict the position Marin resources. Langmuir Circulation is very helpful in case of accidental release of oil spill, as oil buoyant materials are collected by this natural system.

References
1.       Pinet R. Paul, “Invitation to Oceanography”, Jones and Baretlett Publishers,Sudbury, Massachusetts, 8th edition, 2009
2.      Open University (2001), Ocean Circulation (2nd ed.), Butterworth-Heinemann, ISBN 9780750652780
3.      .Scott ,Jon T., Myer ,Glenn E., Stewart ,Ronald. and Walther ,Eric G.” ON THE MECHANISM OF LANGMUIR CIRCULATIONS AND THEIR ROL’E IN EPILIMNION MIXING” Department of Earth and Atmospheric Sciences, State University of New York at Albany,Albany, N.Y. 12203
4.       http://homepages.cae.wisc.edu/~chinwu/CEE514_Coastal_Engineering/2001_Students_Web/Dave_Calkins/Main.html
5.      http://epod.usra.edu/blog/2016/11/langmuir-circulation.html



* Honours, 2013
Part -Time Lecturer in the Department of Geography of Women’s Christian College

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