Lines On Water: Langmuir Circulation- often a nature lover's destination

Tuesday, November 14, 2017

In 1927, as he sailed the Sargasso Sea, future Nobel Laureate Irving Langmuir observed and later explained the mechanics of parallel lines of sargassum floating on the sea surface.  These lines are now known as windrows resulting from Langmuir circulation, named in his honor for his description of the mechanisms that cause the lines to form and be maintained.

 

At least once a year a group of naturalists, most of whom are specialists on birds, gather for a "pelagic" birding trip.  We board a boat somewhere along the coast at about 6 a.m., and head to the blue waters of the open Gulf of Mexico.  The distance of our destination averages some 50-85 miles offshore, and the principal target is birds that typically never come to land in Louisiana – they live over the sea unless nesting.

 

We normally have chance encounters with an array of other wildlife species while underway including bottlenose dolphins, whales, sea turtles, flying fish, ocean sunfish, and much more. 

 

If we see “patches” of natural and/or human-made materials floating on the surface, we speed in that direction.  These patches are actually called windrows, and they are usually parallel lines of “stuff” sometime stretching for more than a kilometer.  In our waters they are composed largely of sargassum, foam/bubbles, and a mixture of floating polyfoam, plastic, and other jetsam blown or dropped off of boats.

 

Other names for windrows include foam lines, rip lines, drift lines, slicks, zones of convergence, and wind streaks.

 

But what causes the formation of these windrows?  Wind for sure, but why do they line up as they do and what keeps the floating matter in nice lines?  Why are they sometimes present, sometimes not?  Why do they form then dissipate as winds stiffen?

 

The answers to these questions, as is often the case in nature, is physics.  As the wind blows across the surface, the friction between wind and water results in a shear force that moves the water in the direction of the wind.  Since winds are always variable – slow, faster, slower – what is called a variable shear force is produced resulting in circular rotation of water in a “cell” which we call Langmuir circulation.  These cells move water in a circle across the surface, down into the water column, back across the bottom, then up again.

Adjacent cell tubes circulate in opposite directions, so they move and arrange objects on the surface into lines (windrows) that are easily visible.  Studies have shown that these lines and their Langmuir circulation zones generally form parallel to the wind or up to 20° to the right of the wind direction and when the water conditions are somewhat calm with winds over the surface being steady and blowing about 6-40 feet per second in speed.  Such winds cause the cells to circulate at speeds of about 1-4 inches per second.

 

Alas, the formation of Langmuir circulation is more complex than simply a response to wind blowing over water.  A number of possible mechanisms have been proposed and studied, and the prevailing theory suggests the following are also important contributing factors:

·      the complicated interaction of capillary waves (ripples moving along the boundary of a fluid – such as the surface – and affected by the presence of surface tension) and,

·      wind-borne films that may be present or absent on the sea’s surface.

 

The result is that anything floating on the surface where Langmuir circulation is occurring is moved to the centerline and kept in place as long as the circulating Langmuir tubes remain formed.  These are our windrows.

 

As stated, windrows may be simple lines of bubbles (see photo), or mats of sargassum mixed with other debris.  Experience has shown that this is the sweet spot for animal observations.  Windrows act as convenient shelters in otherwise open water and are teaming with life, so they attract predators (such as dolphin [fish], sharks, and fishermen), birds that need a rest (bridled terns, phalaropes), sea turtles (those that need to feed, and the tiny ones that need to hide), and a host of small animals that are potential prey (crabs, shrimp, sea horses, pipefish, etc.).

Of course, the predictable circulation patterns here are affecting all sorts of interactions in the ecosystem.  Imagine the affect on distribution of plankton and other organisms floating at the will of water movements.  The circulating cells not only make lines on the surface, but they additionally carry organisms to greater depths then return them to the surface, mixing not only the water but the faunal communities themselves.

So windrows are formed by Langmuir circulation, and their presence is an attractant to wildlife in the sea that seek shelter or prey, birds that need a rest or food, and fishermen who want to catch the predatory fish!

 

Keep your eyes open for foam lines when crossing the Lake Pontchartrain Causeway.  The steady winds often produce nice views of physics in action in our lake.

 

Langmuir windows of foam at the Twin Spans, Lake Pontchartrain, May 3, 2018. Photo by D. A. White.

A windrow of sargassum in the open Gulf Of Mexico40 or so miles south of the mouth of the Mississippi River. September 27, 2009. Photo By Bob Thomas

Sketch of a langmuir winds.

Wondrow of BP-Macando oil mousse (a sticky emulsion of oil and sea water), Gulf Of Mexico about 30 miles S Pass a Loutre, april April 22, 2010.

 the circulation in cells are not perfectly circular Due to the forces and direction of the wind, they are actually spirals. From Open University Course Team, 1989, Fig. 3.26c, Butterworth-Heinamann Ltd.

 Circular movement of the cells prduced in Langmuir circulation. Tajada-Martinez et al, 2011.  J. Appl. Mech. 79(1).