Surfzone eddies: Instabilities or forced by finite-crest-length wave breaking?

Two-dimensional (2D, horizontal) eddies in the surfzone disperse and mix pollutants (fecal indicator bacteria or other tracers such as fine sediments and larvae). Yet, the structure and evolution of the 2D surfzone eddy field is only now starting to come into focus. Surfzone 2D turbulent eddies can be generated through external forcing by finite-crest-length wave breaking or intrinsically through an instability mechanism (i.e., "shear waves"). However, the relative importance of these eddy-generation mechanisms is poorly understood. Surfzone eddies have typically been analyzed in terms of frequency f -alongshore wavenumber k_y spectra. Energy typically lies on a non-dispersive f-k_y ridge, which is consistent with both shear-wave (linear instability) theory and alongshore advection of eddies by the alongshore current. Here, these two eddy generation mechanisms are examined using the wave resolving model funwaveC. The model is applied to the 4 SandyDuck E(f,k_y) examples modeled in Noyes et al. [JGR, 2005] with a wave-averaged NSWE model that only included a shear wave mechanisms (no wave group forcing). The resulting wave-resolving funwaveC modeled E(f,k_y) were much broader and more closely matched the observations than the NSWE model. From the funwaveC model output, the relative contribution of directly forced (extrinsic) versus instability (intrinsic) mechanisms to the surfzone eddy field can be calculated.