Langmuir Circulation

Anyone who has spent time near water has probably seen parallel lines of bubble or other floating debris on a windy day. These are caused by a pattern of water 
movement call Langmuir Circulation - named after the scientist who described the phenomenon back in the 1930s.

Langmuir first noticed the streaks during a trip across the Atlantic. About 600 miles off the coast of New York, he observed long rows of seaweed all oriented in the same direction as the wind. Later, he managed to elucidate the basics of the circulation pattern with a series of experiments on Lake George in New York.

The streaks, which Langmuir called windrows are a visible result of water flowing in a series of rotating spirals with the direction of rotation alternating between spirals. These counterrotating spirals or vortices move water across the surface towards regions of convergence.

Surface water from either direction meet in a line between the vortices and flows downward. Bubbles, seaweed, and other buoyant material that is unable to flow down with the current accumulate at the surface forming the streaks that are characteristic of the circulation pattern.

The counterrotating vortices can occur in sets which results in the multiple, parallel windrows which are always oriented within a few degrees of the wind direction.

Downwelling of water at each line of convergence drives the rotation of the vortices. This movement of water downward at the windrows is balanced by broader more diffuse flows back to the surface in the area between the streaks.

The speed of the surface current is highest in the regions of convergence and is lower on between the windrows. Current speeds also diminish with depth. This variation in flow rates is why the circulation pattern is sometimes described as a corkscrew with the top of the vortices turned downwind towards the regions of convergence.

Langmuir himself determined this was a wind-driven phenomenon, but he did not develop a clear idea of how the wind caused the vortices. In the years after Langmuir’s work, a number of different mechanisms were proposed.

Determining the cause was complicated by the realization that drivers of the circulation pattern differ somewhat between situations, prompting some to suggest that we should make it plural and call it Langmuir circulations because we are actually seeing a set of related phenomena that share similar characteristics.

Even with this complexity, there are some commonalities. In order for Langmuir circulation to occur two things are required. First, there must be surface wind shear - which is just another way to say the speed of the wind-driven surface current diminishes with depth. The other required factor is the presence of wave^*s moving in the same direction as the wind-driven surface current. Waves contribute to the formation of Langmuir circulation via stokes drift.

When most people study surface wave^*s they first learn about the idealized condition where water particles move in perfectly circular paths. Under idealized conditions, there is no net motion of water particles due to wave motion. After a wave passes, all of the individual water particles end up exactly where they began.

Stokes drift describes the common situation where the particles of water move a small amount in the same direction as wave propagation. Since movement associated with surface waves is greatest at the surface and diminishes with depth, the effects of stokes drift are similar to wind shear in that it causes increased movement of water at the surface in the direction the waves are propagating.

In the conditions where Langmuir circulation is found the direction of wave propagation is also the direction of wind-driven surface shear, causing these two forces to reinforce each other.

Modeling studies have shown when small variations in the speed of flow on the surface arise due to a wave breaking or slight variations in the strength of the wind in a small area, wind shear, and wave action reinforce each other causing water at the surface to converge in that area. As the water converges in the area of greatest current, more water moves in behind to fill the space. This creates the movement downwind and towards the region of convergence observed in tracer studies. The converging water gets forced down below the surface creating the downwind and downward jet of water that generates two counter-rotating vortices.

The length of the streaks and the size of the vortices correlate to the size of the waves which why the streaks we see look organized. All the streaks are being generated by the same wind and wave patterns, therefore, share the same orientation and relative size.

While there is some uniformity to the vortices, there is also a lot of dynamism with streaks forming, branching, merging, and disappearing over time.