Transport and accumulation in internal waves

Bands of high plankton concentrations have often been observed moving slowly onshore. These bands of plankton have been inferred to form through the action of vertically swimming plankton interacting with the flows created by high-frequency internal waves. Simple experiments with floating objects (e.g. styrofoam cups) have shown that sometimes the cups move onshore with the bands of high plankton concentration. But sometimes they do not.

We have developed a simple model of a linear, high-frequency internal wave propagating in a two-layered fluid. We placed depth-keeping particles in this flow field, and observed the accumulation and transport patterns. Several points became clear:

• A linear wave cannot transport particles
• The zone of maximum particle concentration is over the trough of the wave, not at the convergence zone
• The bands of high concentration move with the wave, at the same speed as the wave, but the particles do not move with the bands

These dynamics are similar to a traffic jam: the jam can remain stationary (a band of high concentration), but the cars involved in the jam are constantly changing. This is apparent in the animation below.

• Transport and accumulation in an internal wave . One of the particles is circled to show that it doesn't move with the wave. Set your Mpeg player to constantly repeat the animation.

Another version of this model can be seen in the Internal wave banding section of the Patterns in dense phytoplankton blooms page.

So why did some of the styrofoam cups move with the wave bands in the field? It is quite likely that these waves were not linear, but were non-linear internal waves. Ongoing work with my graduate student Cleridy Lennert is aimed at elucidating the relative abilities of linear and nonlinear waves to transport particles.

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