Ann E. Gargett

Local isotropy and the decay of turbulence in a stratified fluid

The validity of the assumption of local isotropy is investigated using measurements of three orthogonal components of the turbulent velocity fields associated with initially high-Reynolds-number geophysical turbulence. The turbulent fields, generated by various large-scale internal motions caused by tidal flows over an estuarine sill, decay under the influence of stable mean density gradients. With measurements from sensors mounted on a submersible, we examine the evolution of spectral shapes and of ratios of cross-stream to streamwise components, as well as the degree of high-wavenumber universality, for the observational range of the parameter I ≡ k s / k b = l b / l s . This ratio is a measure of separation between the Kolmogoroff wavenumber k s ≡ (e/ν 3 ) ¼ ≡ 2π/ l s typical of scales by which turbulent kinetic energy has been dissipated (at rate e), and the buoyancy wavenumber k b ≡ ( N 3 /e) ½ ≡ 2π/ l b typical of scales at which the ambient stratification parameter N ≡ (− g ρ z /ρ 0 ) ½ becomes important. For values of I larger than ∼ 3000, inertial subranges are observed in all spectra, and the spectral ratio ϕ 22 /ϕ 11 of cross-stream to streamwise spectral densities reaches the isotropic value of 4/3 for about a decade in wavenumber. As k s /k b decreases, inertial subranges vanish, but spectra of the cross-stream and streamwise components continue to satisfy isotropic relationships at dissipation wavenumbers. We provide a criterion for when e may safely be estimated from a single measured component of the dissipation tensor, and also explore questions of appropriate low-wavenumber normalization for buoyancy-modified turbulence.

Mixing in the Romanche Fracture Zone

The Romanche Fracture Zone is a major gap in the Mid-Atlantic Ridge at the equator, which is deep enough to allow significant eastward flows of Antarctic Bottom Water from the Brazil Basin to the Sierra Leone and Guinea Abyssal Plains. While flowing through the Romanche Fracture Zone, bottom-water properties are strongly modified due to intense vertical mixing. The diapycnal mixing coefficient in the bottom water of the Romanche Fracture Zone is estimated by using the finestructure of CTD profiles, the microstructure of high-resolution profiler data, and by constructing a heat budget from current meter data. The finestructure of density profiles is described using the Thorpe scalesLT. It is shown from microstructure data taken in the bottom water that the Ozmidov scale LO is related to LT by the linear relationship LO 5 0.95LT, similar to other studies, which allows an estimate of the diapycnal mixing coefficient using the Osborn relation. The Thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream (eastward) of the main sill of the Romanche Fracture Zone. In this region, a mean diapycnal mixing coefficient of about 1000 3 1024 m2 s21 is found for the bottom water. Estimates of cross-isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the Romanche Fracture Zone and the nearby Chain Fracture Zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the Romanche Fracture Zone. Although the two fracture zones occupy only 0.4% of the area covered by the Sierra Leone and Guinea Abyssal Plains, the diffusive heat fluxes across the 1.4 8C isotherm in the Romanche and Chain Fracture Zones are half that found over the abyssal plains across the 1.88C isotherm, emphasizing the role of these passages for bottom-water property modifications.

A Composite Spectrum of Vertical Shear in the Upper Ocean

Abstract Results from three separate velocity profilers operated nearly simultaneously in the northwest Atlantic in 1975 are used to form a composite shear spectrum over vertical wavelengths from 100 m down to a few centimeters. This exercise constitutes an intercomparison of the three different measurement techniques and reveals a shear spectrum which is approximately fiat at a WKB-scaled level from k = 0.01 cpm through k0 ≈ 0.1 cpm, then falls as k−1 to a buoyancy wavenumber k0 = (N3/ϵ)1/2 determined by the local average Vaisala frequency N and the volume-averaged dissipation rate ϵ. Various consequences of the observed shear spectral shape are explored.

The diffusive regime of double-diffusive convection

The diffusive regime of double-diffusive convection is discussed, with a particular focus on unresolved issues that are holding up the development of large-scale parameterizations. Some of these issues, such as interfacial transports and layer-interface interactions, may be studied in isolation. Laboratory work should help with these. However, we must also face more difficult matters that relate to oceanic phenomena that are not represented easily in the laboratory. These lie beneath some fundamental questions about how double-diffusive structures are formed in the ocean, and how they evolve in the competitive ocean environment.

On the interaction of surface and internal waves

Abstract : The steady-state interaction between surface waves and long internal waves is investigated theoretically using the radiation stress concepts derived by Longuet-Higgins + Stewart or Phillips (1966).

Mixing Efficiencies in Turbulent Tidal Fronts: Results from Direct and Indirect Measurements of Density Flux

Abstract The authors report direct measurements of density flux at a single depth in a turbulent tidal flow, made by towing a CTD beside the vertical beam of a modified acoustic Doppler current profiler. The direct flux estimates are compared with indirect estimates of density flux based on simultaneous microscale profiler measurements of ϵ and χ, dissipation rates of turbulent kinetic energy and of temperature variance, respectively. Two mixing efficiency estimates are made using Γd from the ratios of indirect flux estimates and Γo from the ratio of direct to ϵ flux estimates. The analysis indicates that • Γd is no different from that determined in other open ocean experiments and is independent of the sign of the flux • Γ d 0) < |Γo| (flux < 0). The consequences for interpretation of ocean microstructure flux estimates are discussed.

A Microstructure Instrument for Profiling Oceanic Turbulence in Coastal Bottom Boundary Layers

Abstract A free-falling instrument has been built to measure temperature, salinity and turbulent shear from near surface to within 15 cm of the ocean bottom. A probe guard mounted at the lower end of the instrument protects sensitive shear and temperature sensors from bottom sediments. The noise level in the shear signal corresponds to a dissipation rate of 3.0 × 10−7 W m−3, with vibrations of the probe guard providing the largest component of the noise. The signals are transmitted through a neutrally buoyant line to the ship where they are displayed in real time and recorded for later analysis. The profiling technique is capable of 20 profiles per hour through 50 m of water. Simultaneous profiles of turbulent microstructure and density can now be consistently obtained through the entire water column.

Evolution of scalar spectra with the decay of turbulence in a stratified fluid

Temperature measurements taken in association with the velocity measurements described by Gargett, Osborn & Nasmyth (1984) are examined. With careful noise removal the temperature dissipation spectrum is fully resolved and χ, the dissipation rate of temperature-fluctuation variance, is determined directly. With directly measured values of χ and e, the turbulent-kinetic-energy dissipation rate per unit mass, the observed temperature spectra are non-dimensionalized by Oboukov–Corrsin–Batchelor scaling. Shapes and levels of the resulting non-dimensional spectra are then examined as functions of the degree of isotropy (measured) in the underlying velocity field. Two limiting cases are identified: Class A, associated with isotropic velocity fields; and Class B, associated with velocity fields which are anisotropic (owing to buoyancy forces repressing vertical relative to horizontal dimensions of energy-containing ‘eddies’). The present observations suggest that the Corrsin–Oboukov–Batchelor theory does not provide a universal description of the spectrum of temperature fluctuations in water. Class A scalar spectra have neither

Observing Turbulence with a Modified Acoustic Doppler Current Profiler

k^{-\frac{5}{3}}

A Global Ocean Model with Double-Diffusive Mixing

nor k −1 subranges: a Batchelor-spectrum fit to the high-wavenumber roll-off region yields a value of 12 for the ‘universal’ constant q . In striking contrast, the buoyancy-affected Class B spectra exhibit a clear