Observations of internal solitary waves near the Oregon coast with an inverted echo sounder

Abstract

Observations of internal solitary waves (ISWs) were made near the Oregon coast with a moored, 200‐kHz inverted echo sounder (IES) and a 12‐element vertical array of thermistors over a 14‐day period during September 1995. Downward pycnocline displacements conforming to the classical Kortweg‐deVries, hyperbolic secant squared profile, with typical amplitudes of 5 to 14 m, propagation speeds of 0.4 to 0.6 m s−1, and duration of 100 to 200 s were observed with both the IES and thermistor array. Pycnocline displacements from 5 to 22 m depth were observed acoustically exploiting the backscatter from Zooplankton layers, which acted as tracers of water motions. The typical volumetric scattering strength of these Zooplankton layers was near −75 dB (referenced to 1 m−1). Within the top 10 m, plumes of microscopic air bubbles injected by breaking surface waves were advected downward by the ISWs, with volumetric scattering strengths in excess of −36 dB. Finally, near‐surface acoustic scattering zones suspected to exhibit backscatter from turbulent microstructure were observed, with typical volumetric scattering strength near −68 dB. These near‐surface zones were coincident with model predictions of large shear instabilities and local gradient Richardson numbers below the critical value for generation of turbulence.