Mode 2 waves on the continental shelf: Ephemeral components of the nonlinear internal wavefield

Shoreward propagating, mode 2 nonlinear waves appear sporadically in mooring records obtained off the coast of New Jersey in the summer of 2006. Individual mode 2 packets were tracked between two moorings separated by 1 km; however, packets could not be tracked between moorings separated by greater distances from one another (similar to 10 km). The inability to track individual packets large distances through the mooring array combined with detailed observations from a ship suggest that these waves are short lived. The evolution of the ship-tracked wave group was recorded using acoustic backscatter, acoustic Doppler current profilers, and turbulence profiling. The leading mode 2 wave quickly changed form and developed a tail of short, small-amplitude mode 1 waves. The wavelength of the mode 1 oscillations agreed with that expected for a copropagating tail on the basis of linear theory. Turbulent dissipation in the mixed layer and radiation of the short mode 1 waves contributed to rapid energy loss in the leading mode 2 wave, consistent with the observed decay rate and short life span of only a few hours. The energy in the leading mode 2 wave was 10-100 times smaller than the energy of mode 1 nonlinear internal waves observed during the experiment; however, the magnitudes of wave-localized turbulent dissipation were similar.