Observations of the Near-Surface Internal Wavefield

Abstract

During January 1977 a cruise was conducted off the California coast on the Research Platform FLIP. Repeated temperature profiling devices were used to sense the internal wavefield in the top 400 m of the sea. From a sequence of 8192 profiles, vertical-velocity spectra and vertical coherence were calculated. Near-surface coherence was found to increase with increasing frequency between local inertial and Väisälä frequencies. Below 200 m the coherence was approximately constant with frequency. The near-surface change in the vertical coherence patterns results from the selective attenuation of the longer vertical wavelengths as the surface is approached. From frequency-depth changes in the near-surface coherence, variations of the internal-wave spectral form can be inferred, in spite of the fact that the deep vertical coherence remains constant. This near-surface effect is not so apparent in data from horizontal-velocity sensors, as only the vertical component of motion is constrained to vanish at the sea surface.