The Basics of Langmuir Circulation

Introduction

This page is intended to provide the reader with some basic information about Langmuir circulation.  Sections include discussions of what it is, formation of the cells, and applications/areas of interest.  Finally, there are some links for those looking for more information than presented here.

What is Langmuir Circulation?
Who discovered it?
How do Langmuir cells form?
Why is anyone interested?
Where can I learn more?
Who the heck wrote this travesty?
 


What is Langmuir Circulation?

Surface Streaks If you live near any body of water you may have already noticed the tell-tale streaks of Langmuir circulation.  Those signs are streaks of floating material or bubbles that collect on the water surface.  These streaks align with thewind directions, though there is often a slight deviation of 0-20 degrees.At right you can see a photo (Alex Smith) of streaks off of South Hampton. Often these streaks are referred to as windrows.
 
 
 
 
 

Cross section of circulation
The image at left not only demonstrates my less-than-stellar artistic ability, but also shows the mechanism for formation of these streaks.  Imagine the wind is blowing directly out of your monitor.  The black circles represent long tubes, or cells of rotating water pointed in the same direction as the wind.  As this water circulates past the surface, it catches material (in green) and transports it.  Notice that the cells rotate in opposite directions, and so material from two cells is brought together, clearing the rest of the water surface.  Along the line where the cells meet, they circulate downwards.  However, any buoyant (floating) material is forced to stay at the surface- the buoyancy counteracts the down-welling velocity, and thus the surface streaks are formed.  For an excellent animation of this process, click here.

Characteristics

The typical depth of langmuir cells (i.e. radius of the black circles above) is 4-6 m.  However, certain effects can be observed up to 200 m below the water surface.  The cells can be spaced anywhere from 10-50 meters apart.  The length of the cells have been observed to be anywhere from a few meters long, to many kilometers into the ocean.  The cell axes are typically aligned along the wind direction, but may vary by as much as 20 degrees.  When wind direction changes, the cells will gradually shift to align with the new direction, lagging behind by 15-20 minutes.

Velocities within the cell are only a fraction of the wind velocities that create them.  However, downwelling velocities can reach up to 0.2 m/s, important in some applications.


Who discovered it?

Lan gmuir circulation gets its name from the first person to study the phenomenon.  Irving Langmuir noticed patterns of floating seaweed, Sargassum when crossing the Atlantic in 1938.  Intrigued, he conducted experiments in a lake in order to explain the formation of these strips.


How do Langmuir cells form?
Langmuir cells can be seen in oceans, seas, lakes, and even rivers.  There have been a number of theories explaining the formation of Langmuir cells.  The generally accepted model points to the role of Stoke's drift*.  The process occurs as follows:

               1.  Wind blows across the surface of water, creating a surface shear force.
               2.  Minor variations in the wind result in a variable shear force.
               3.  Variable shear force creates vertical rotating cells of water (perpendicular to the water surface).
               4.  Stoke's drift begins to shift the rotating cells.
               5.  The top portion of the cell moves downwind until the entire cell is horizontal.

*Stoke's drift:  Consider a body of water as multiple small layers.  As wind passes over the water surface, it places a shear stress on the top layer.  That layer then applies a slightly smaller shear stress on the next layer down.  In this way, a surface stress will result in a greater transport near the surface than near the bottom.

This is a simplified explanation of the CL2 process.  The CL2 process is the second formation theory put forth by Craik and Leibovich.  Alex Smith has drawn an excellent representation of this process.  Check it out on his site.

Wind speeds must typically reach 3 m/s in order to generate these Langmuir cells.  In addition, if water conditions are turbulent cells may not be able to develop at all..  At windspeeds greater than 13 m/s, instabilities start to dominate and cells disintegrate, amalgamate, or regenerate.  Cells have been observed forming Y-junctions.  These are typically at angles of 30 degrees, and point in the direction of the prevalent wind.

Research seems to indicate that this model cannot totally explain the circulation observed in the field.  Some researchers believe that heat convection and exchange can play a role in forming/damping existing cells.  In fact, a recent paper by Li & Garret investigate the effects of heating on circulation patterns.



Why is anyone interested?

There are many reasons (beyond pure science) to understand the patterns of circulation.  These include:

                1.  Mixed layer implications
                2.  Biological systems
                3.  Particle tracking

Mixed Layer

The mixed layer is the upper stratus of the ocean where temperature and density are relatively uniform.  Immediately below this mixed layer, there is a significant gradient to other densities/temperatures.  Although it is unclear exactly how Langmuir Circulation impacts the mixed layer, a study by Li, Zahariev, & Garret found that that the mixed layer can be deepened up to a depth of 200m in the presence of Langmuir cells.  In addition, effects can be witnessed within anywhere from a few minutes to several hours after cells develop.

Biological Systems

Some studies have investigated the interaction of Langmuir cells and their circulation patterns with biological systems, like plankton.  Circulation patterns can cause real plankton distribution to differ signifcantly from predicted results.  In particular, plankton 'patchiness' can be seen (i.e. distinct clouds of plankton).  Plankton have also been observed remaining in the same position, relative to a fixed feature.  This raises questions about what sort of evolutionary adavantage might the plankton be benefit from in exchange for the energy expended to 'hold position'.

Particle Tracking

Of additional interest is the potential to accurately track and predict the positions of groups of particles, using an understanding of Langmuir circulation patterns.  For instance, oil- a buoyant material, can potentially collect on the surface of the water in the long windrows, seen before with bubbles.  In the case of accidental release (i.e. spills), cleanup efforts could be made easier by this natural collection system.  Studies have found that cross wind diffusion is about 20% of the level expected when no circulation is present.  Unfortunately, if oil droplets are small enough (10-100 microns) down-welling velocites can overcome their buoyancy and disperse them into the water column.  Additionally, most studies have so far worked on the assumption that Langmuir cells are perfectly aligned with the prevalent wind, something that is not always the case.



Where can I learn more?

Hopefully this page has answered some of your questions, but perhaps you want to know more...

These sites provide useful information or graphics on Langmuir circulation, or other coastal phenomena.

Discussion of Langmuir Circulation Papers  This page holds a number of references that study applications of Langmuir circulation.  If you are looking for more in depth discussion check these out.  You will need Ghostscript to read the full papers, however.

Alex Nimmo Smith's Page  Alex Smith has put together a very nice, informative page, with some really nifty animations.  Go take a look.

Great Lakes Information  Grand Valley State University's page includes information on topics (including Langmuir circulation) dealing with the Great Lakes.  A good way to introduce yourself to something new.



Who the heck wrote this travesty?

My name is David Calkins.  I am an undergraduate in Civil Engineering at the University of Wisconsin-Madison.  In my last semester, I took professor Chin Wu's Coastal Engineering course.  Part of the coursework is a project consisting of a presentation and web page- exactly what you see here.  If you have any questions or complaints, feel free to email me at calkinsdo@yahoo.com.