Internal wave field generated by oscillating rough topography

Internal waves generated by oscillating topography with a series of ridges in a stratified medium are experimentally explored. Experiments represent oscillating tidal flow in the ocean where small-scale roughness on topography cannot be fully resolved in global circulation models, but the generated internal wave field can impact global mixing and ocean dynamics. Here, the influence of topography roughness is evaluated by including different numbers of ridges, with slopes equivalent to the edge slope of the full topography, on top of the original topography. Specifically, the internal wave field generated by a wide plateau shape is compared with the same shape except with three to six Gaussian ridges overlain on the plateau. In all scenarios, a complex pattern of internal waves generated by each ridge is observed. However, the results show as the number or width of ridges increases, the waves generated by the ridges near the center of the plateau decay very quickly and in the far field the internal wave field is indistinguishable from that generated by a smooth plateau. A non-dimensional number is suggested that accounts for both the number of ridges and overall topography width while defining a limit for which plateau-like internal wave generation is expected and this form of surface roughness may be neglected.