The movie on the right shows three different cases of shear instability. In each cases instability is generated by gradients in the fluid velocity. This instability is counteracted by the stabilising effect of density stratification. The subtle balance between velocity and density can have a big effect -- this can be seen by the qualitative difference between the three panels.

The top panel shows classical Kelvin-Helmholtz billows, which wrap up the density interface and produce secondary turbulence. The lower panel is a case of Holmboe's instability -- a weaker instability which is difficult to observe. The centre panel is a mix of the two instabilities.

Shear instability is important in the ocean because it can generate turbulence which contributes to the mixing of different oceanic waters. For more information see:

A. McC. HOGG & G. N. IVEY, (2003). The Kelvin-Helmholtz to Holmboe instability transition in stratified exchange flows. J. Fluid. Mech. 477, 339-362. [On-line Copy from JFM]