Vortex dynamics beneath a surfactant-contaminated ocean surface

Data of a vorticity field from direct numerical simulation of the subsurface turbulent shear flow near a surfactant-contaminated ocean surface are analyzed, The aim of the study is to understand the coupled interaction processes among the surface renewal eddies, the surfactant, and the interface. It is found that as a subsurface coherent intense eddy approaches the interface, such as the identified rising horseshoe vortex, a strong counterrotating spanwise vortex is induced at the interface in the early stage of the interaction. This surface vortex acts as the vortex shed from a no-slip boundary to repel the approaching eddy, The competition between the vortex stretching owing to the inviscid blocking effect of the interface and the viscous dissipation and diffusion causes the surface-parallel portion of the approaching subsurface vortex to disconnect. In the latter stage of the interaction, the surviving surface-normal portion of the underlying vortex continues to redistribute the surfactant and induces additional surface vortices. Viscous diffusion, which is the only production mechanism of vertical vorticity flux near a contaminated interface, eventually results in the disconnected underlying vortex reconnecting with the interface. Enstrophy transport between the surface-connected vortices and the latterly induced surface vortices dominate the enstrophy budget in the immediate vicinity of the contaminated interface. The identified interaction scenario between the surfactant-contaminated interface and the subsurface vortical flow can exist at neither a no-slip nor a free-slip surface.