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 EES 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. Copyright 1998 by the American Geophysical Union.