Signatures of surface wave/internal wave interactions: Experiment and theory

Abstract

This paper summarizes results from an extended investigation of the physics of surface wave/internal wave interactions. During summer conditions of stratification, internal solitary waves are generated by the semidiurnal tide at the continental shelf break off New York. The waves evolve as packets of solitons having rank-ordered amplitudes, wave spacings, and crest lengths. They propagate toward shore and are ultimately dissipated in the region where the mixed layer intersects the bottom. We have studied their hydrodynamic properties, their interactions with surface waves and with each other, and the scattering of electromagnetic radiation from their accompanying surface roughness variations. Measurement using a towed thermistor chain. CTD casts, and moored current meter arrays have allowed a detailed characterization of the subsurface and near-surface hydrodynamics. Quantitative surface signatures were obtained using: (a) TV Stilwell methods, (b) calibrated airborne synthetic aperture radar at L- and X-bands, and (c) the L-band SIR-B synthetic aperture radar on the space shuttle Challenger. The scattering data have been analyzed using the wave action equation for calculating internal current/surface wave interactions. Hughes' description of the wave relaxation rate has been used to obtain the perturbation to the surface wave spectrum from the internal currents and current straining. Radar backscatter variations from the perturbed surface wave roughness were then calculated using a variant of the Kirchhoff approximation to the scattering integral. Both Bragg scattering and specular point scattering are important in this case. Excellent agreement is obtained between the theory and the observations.