Perspective View of Frequency Downshifting

Perspective View of a Spilling Breaking Wave. The velocity and time are normalized based on the wavenumber and the phase speed. The results are periodically extended parallel and transverse to the wind. The 0.5 isosurface of the volume fraction is shown. The view is a perspective view looking down on the ocean surface.

Initially there are four waves. As the waves break, a frequency downshift occurs, until toward the end of the simulation there are only two waves. The downshift appears to be primarily due to a combination of a Benjamin-Feir instability and wave breaking.

Monopolar, dipolar, tripolar, and quadrupolar vortices are shed out the back of the spilling breaking wave. The intense vortical structures form knots on free surface. The shedding of coherent vortical structures is consistent with an inverse energy cascade. The physics of knots and upwellings are different. Knots and upwellings both occur in the wake of spilling breaking waves. Knots form when concentrated vorticity connects normal to the free surface. Upwellings are caused by vortex rings. The scrubbing of foam in small nearly circular patches is the surface signature of upwellings. Another animation in this Playlist shows the vortex rings that form beneath spilling breaking waves.