The subject of ocean turbulence is in a state of discovery and development with many intellectual challenges. This book describes the principal dynamic processes that control the distribution of turbulence, its dissipation of kinetic energy and its effects on the dispersion of properties such as heat, salinity, and dissolved or suspended matter in the deep ocean, the shallow coastal and the continental shelf seas. It focuses on the measurement of turbulence, and the consequences of turbulent motion in the oceanic boundary layers at the sea surface and near the seabed. Processes are illustrated by examples of laboratory experiments and field observations. The Turbulent Ocean provides an excellent resource for senior undergraduate and graduate courses, as well as an introduction and general overview for researchers. It will be of interest to all those involved in the study of fluid motion, in particular geophysical fluid mechanics, meteorology and the dynamics of lakes.

https://assets.cambridge.org/97805218/35435/frontmatter/9780521835435_frontmatter.pdf

The Turbulent Ocean

Detailed observations of the structure within internal solitary waves propagating shoreward over Oregon's continental shelf reveal the evolving nature of interfaces as they become unstable and break, creating turbulent flow. A persistent feature is high acoustic backscatter beginning in the vicinity of the wave trough and continuing through its trailing edge and wake. This is demonstrated to be due to enhanced density microstructure. Increased small-scale strain ahead of the wave trough compresses select density interfaces, thereby locally increasing stratification. This is followed by a sequence of overturning, high-density microstructure, and turbulence at the interface, which is coincident with the high acoustic backscatter. The Richardson number estimated from observations is larger than 1/4, indicating that the interface is stable. However, density profiles reveal these preturbulent interfaces to be O(10 cm) thick, much thinner than can be resolved with shipboard velocity measurements. By as...

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Structure and Generation of Turbulence at Interfaces Strained by Internal Solitary Waves Propagating Shoreward over the Continental Shelf

[1] A sequence of three internal solitary waves of elevation were observed propagating shoreward along a near-bottom density interface over Oregon's continental shelf. These waves are highly turbulent and coincide with enhanced optical backscatter, consistent with increased suspended sediments in the bottom boundary layer. Non-linear solitary wave solutions are employed to estimate wave speeds and energy. The waves are rank ordered in amplitude, phase speed, and energy, and inversely ordered in width. Wave kinetic energy is roughly twice the potential energy. The observed turbulence is not sufficiently large to dissipate the waves' energy before the waves reach the shore. Because of high wave velocities at the sea bed, bottom stress is inferred to be an important source of wave energy loss, unlike near-surface solitary waves. The wave solution suggests that the lead wave has a trapped core, implying enhanced cross-shelf transport of fluid and biology.

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Internal solitary waves of elevation advancing on a shoaling shelf

High-frequency acoustic scattering techniques have been used to investigate dominant scatterers in mixed zooplankton populations. Volume backscattering was measured in the Gulf of Maine at 43, 120, 200, and 420 kHz. Zooplankton composition and size were determined using net and video sampling techniques, and water properties were determined using conductivity, temperature, and depth sensors. Dominant scatterers have been identified using recently developed scattering models for zooplankton and microstructure. Microstructure generally did not contribute to the scattering. At certain locations, gas-bearing zooplankton, that account for a small fraction of the total abundance and biomass, dominated the scattering at all frequencies. At these locations, acoustically inferred size agreed well with size determined from the net samples. Significant differences between the acoustic, net, and video estimates of abundance for these zooplankton are most likely due to limitations of the net and video techniques. No other type of biological scatterer ever dominated the scattering at all frequencies. Copepods, fluid-like zooplankton that account for most of the abundance and biomass, dominated at select locations only at the highest frequencies. At these locations, acoustically inferred abundance agreed well with net and video estimates. A general approach for the difficult problem of interpreting high-frequency acoustic scattering in mixed zooplankton populations is described.

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Determining dominant scatterers of sound in mixed zooplankton populations

Extending the range of microstructure measurements from the upper thermocline of the open ocean to shallow waters near shore and to the abyss has greatly increased in the range of turbulent intensities being observed. As a result, it is necessary to reexamine the adequacy of microstructure probes to resolve dissipation-scale gradients of velocity and temperature. These variances are needed to estimate directly the viscous dissipation rate, ϵ, with airfoil probes and the diffusive dissipation rate, χT, with thermistors.

Uncertainties and Limitations in Measuring ϵ and χT

Acoustic backscatter has produced spectacular images of internal ocean processes for nearly two decades, but interpretation of the images remains ambiguous because several mechanisms can generate measurable backscatter. The authors present what is thought to be the first simultaneous measurements of calibrated acoustic returns and turbulent microstructure, collected in a set of 20-m-tall billows. The observations are from Admiralty Inlet, a salt-stratified tidal channel near Puget Sound. Scattering due to turbulent microstructure alone is strong enough to explain the measured backscatter at specific sites within the billows. Existing formulations underestimate the strength of acoustic backscatter from turbulent microstructure. Due to a misinterpretation of the high-wavenumber temperature spectrum, some previous formulations underestimate the differential Scattering cross section (σ) when scattering from the viscous-convective subrange. Also, the influence of salinity on refractive-index fluctuati...

R. T. Miyamoto

Papers

Acoustic Backscatter from Turbulent Microstructure