Abstract:
In this paper, measurements of high-frequency
broadband (160–590 kHz) acoustic backscattering from surface
trapped nonlinear internal waves of depression are presented.
These waves are ideal for assessing the contribution from oceanic
microstructure to scattering as they are intensely turbulent.
Almost coincident direct microstructure measurements were performed
and zooplankton community structure was characterized
using depth-resolved net sampling techniques. The contribution to
scattering from microstructure can be difficult to distinguish from
the contribution to scattering from zooplankton using a single
narrowband frequency as microstructure and zooplankton are
often colocated and can have similar scattering levels over a range
of frequencies. Yet their spectra are distinct over a sufficiently
broad frequency range, allowing broadband backscattering measurements
to reduce the ambiguities typically associated with the
interpretation of narrowband measurements. In addition, pulse
compression signal processing techniques result in very high-resolution
images, allowing physical processes that are otherwise
hard to resolve to be imaged, such as Kelvin–Helmholtz shear
instabilities. In this study, high-resolution acoustic observations of
multiple nonlinear internal waves are presented and regions with
distinct scattering spectra are identified. Spectra that decrease in
level across the available frequency band were highly correlated
to regions of intense turbulence and high stratification, and to
Kevin–Helmholtz shear instabilities in particular. Spectra that
increase in level across the available frequency band were consistent
with scattering dominated by small zooplankton. Simple
inversions for relevant microstructure parameters are presented.
Limitations of, and improvements to, the broadband system and
techniques utilized in this study are discussed.
