The steady undertow created by waves breaking at a beach and slowly flowing offshore can become unstable and create a train of submerged offshore migrating vortices with shorter length scales and longer time scales than the incident waves, as shown by Matsunaga, Takehara & Awaya (1988, 1994). These vortices rotate about horizontal axes parallel to the shoreline. Our larger-scale laboratory experiments show that an additional layer of vortices can exist over the water depth, with vortices near the water surface rotating in the same direction as the wave-induced water particle trajectories, while those located at about mid-depth rotate in the opposite direction.

A theoretical and numerical analysis shows that these vortices are due to instabilities of the undertow. Far offshore of the surf zone, the vortex trains decay because the velocity profile for the undertow becomes linear over depth, hence neutrally stable to any disturbances.