Measurements in the South China Sea reveal the structure of the bottom boundary layer beneath onshore propagating highly nonlinear internal solitary waves of depression. Offshore directed free stream velocities beneath thirteen waves with durations of 10-20 minutes and velocities up to 1.4 m/s are consistent with the solitary wave solution to the Korteweg de Vries equation, as are phase velocities estimated from pressures and free stream velocities. The measurements indicate a thin layer during free stream acceleration, a thicker layer with an inflected velocity during deceleration, and a long lived sediment laden wake, after wave passage in the free stream, with velocities opposite those at maximum flow strength. Reynolds averaged Navier Stokes simulations with the k-ε turbulence model reproduce the measured velocities and turbulent Reynolds shear stresses accurately during acceleration and early deceleration. However, differences between the simulations and the measurements during late deceleration and in the wake suggest energetic large scale turbulence not represented by the simulations. This turbulence might be similar in origin to coherent vortices that have been observed in laboratory experiments and direct numerical simulations at much smaller Reynolds numbers, which have been attributed to absolute and global instabilities resulting from inflected velocity profiles.

Oceanic internal gravity waves propagate along density stratification within the water column and are ubiquitous. They can propagate thousands of kilometers before breaking in shoaling bathymetry and the ensuing turbulent mixing affects coastal processes and climate feedbacks. Despite their importance, internal waves are intrinsically difficult to detect as they result in only minor amplitude deflection of the sea surface; the need for global detection and long time series of internal waves motivates a search for geophysical detection methods. The pressure coupling of a propagating internal wave with the sloping seafloor provides a potential mechanism to generate seismically observable signals. We use data from the South China Sea where exceptional oceanographic and satellite time series are available for comparison to identify internal wave signals in an onshore passive seismic dataset for the first time. We analyze potential seismic signals on broadband seismometers in the context of corroborating oceanographic and satellite data available near Dongsha Atoll in May-June 2019 and find a promising correlation between transient seismic tilt signals and internal wave arrivals and collisions in oceanic and satellite data. It appears that we have successfully detected oceanic internal waves using a terrestrial seismometer. This initial detection suggests that the seismic detection and amplitude determination of oceanic internal waves is possible and can potentially be used to expand the historical record by capitalizing on the existing terrestrial seismic network.