Observations of two-dimensional turbulence in the surfzone

Low-frequency, many-minute-period horizontal surfzone eddies are an important mechanism for the dispersion of material, transporting larvae, pollutants, sediment, and swimmers both across and along the nearshore. Previous numerical, laboratory, and field observations on alongshore uniform bathymetry with no or roughly uniform mean background flows suggest that the low-frequency eddies may be the result of a two-dimensional inverse energy cascade that transfers energy from relatively small spatial-scale vorticity injected by depth limited breaking waves to larger and larger spatial scales. Here, using remotely sensed high-spatial resolution estimates of currents, those results are extended to surfzones with strong complex mean circulation patterns [flows O(1 m/s)] owing to nonuniform bathymetry. Similar to previous results, wavenumber spectra and second-order structure functions calculated from the observations are consistent with a two-dimensional inverse energy cascade. The size of the largest eddies is shown to depend on the surfzone width and the spatial scales of the mean currents. Third-order structure functions also are consistent with an inverse cascade for spatial scales greater than & SIM;50 m. At smaller scales, the third-order structure functions suggest a mixture of inverse and forward cascades.

Automated summary

Low-frequency horizontal surfzone eddies that last for several minutes are crucial for dispersing materials, such as larvae, pollutants, sediment, and swimmers, both across and along the nearshore. Previous studies have suggested that these eddies may be a result of a two-dimensional inverse energy cascade, transferring energy from small spatial-scale vorticity to larger scales. The current study extends these findings to surfzones with nonuniform bathymetry and complex mean circulation patterns, confirming the presence of an inverse energy cascade.