Barry Ruddick

Two Years in the Life of a Mediterranean Salt Lens

Abstract A lens of Mediterranean water (Meddy) was tracked in the eastern North Atlantic for two years with SOFAR floats. The Meddy was first found between the Canary Islands and the Azores in October 1984. It center moved in an irregular pattern, at speeds of a few cm s−1, and translated 1100 km to the south in two years. This Meddy was surveyed four times by CTD and velocity profilers, and once with the microstructure profiler EPSONDE. When observed during the first two surveys the Meddy had a core that was stably and smoothly stratified in both salinity and temperature, nearly uniform in the horizontal, and was saltier than the surrounding ocean by 0.65 psu. The Meddy was eroded from its edges, top and bottom, and lost salt and hat with an e-folding time of about one year. The salinity at the center remained at its original value during the first year and decreased during the second year. Evidence was seen for mixing by lateral intrusions, double diffusion, and turbulence; the intrusions are thought to...

Maximum likelihood spectral fitting: The Batchelor spectrum

Abstract A simple technique for fitting spectra that is applicable to any problem of adjusting a theoretical spectral form to fit observations is described. All one needs is a functional form for the theoretical spectrum and an estimate for the instrumental noise spectrum. The method, based on direct application of the maximum likelihood approach, has several advantages over other fitting techniques. 1) It is unbiased in comparison with other least squares or cost function–based approaches. 2) It is insensitive to dips and wiggles in the spectrum, due to the small number of fitted parameters. It is also robust because the range of wavenumbers used in the fit is held fixed, and the built-in noise model forces the routine to ignore the spectrum as it gets down toward the noise level. 3) The method provides a theoretical estimate for error bars on the fitted Batchelor wavenumber, based on how broad or narrow the likelihood function is in the vicinity of its peak. 4) Statistical quantities that indicate how w...

Intrusive Mixing in a Mediterranean Salt Lens—Intrusion Slopes and Dynamical Mechanisms

Abstract Thermohaline intrusions found in Meddy “Sharon” were laterally coherent The slope of these intrusions across density surfaces was in opposite senses in the upper and lower part of the meddy, consistent with (i) the McIntyre instability for Prandtl number less than one, in which case mass flux (assumed equal in heat and salt diffusivity) dominates over viscosity, or (ii) double-diffusive intrusions, in which diffusive fluxes dominate in the upper, diffusively unstable portion of the meddy, and finger fluxes dominate in the lower, fingering-unstable part. The magnitudes of the intrusion slopes are outside the range of angles for which the McIntyre mechanism can provide energy to the intrusive motions, but within the range of angles for which double-diffusive mixing can drive the intrusions. Thus, the energy source for the intrusions is not the kinetic or potential energy of the geostrophic shear flow, but is the potential energy associated with the lateral salinity and temperature gradients. Furthe...

Variations in Apparent Mixing Efficiency in the North Atlantic Central Water

Abstract Microstructure data from the North Atlantic Tracer Release Experiment (NATRE) are presented, providing detailed profiles of the thermal variance χ in the upper 360 m of the Canary Basin for the fall and spring seasons. The Osborn–Cox model is used to compute the diffusivity KT. The diffusivity for the depth range 240–340 m is found to be 1.0(±0.04) × 10−5 m2 s−1 in the fall and 2.2(±0.1) × 10−5 m2 s−1 in the spring, in good agreement with dye-inferred diffusivities at similar depths. Measured turbulent kinetic energy (TKE) dissipation rates were found to be contaminated by hydrodynamic noise, so the Osborn dissipation method was not used to compute Kρ. However, data segments for which the TKE dissipation rate (e) was large enough to be unaffected by noise were used to compute the “apparent mixing efficiency” Γd. The computed Γd values are used to investigate variations in apparent mixing efficiency with respect to density ratio (Rρ) and turbulence Reynolds number [e/(νN2)], in an attempt to eluci...

Evolution of a mediterranean salt lens : scalar properties

Abstract The evolution of a Mediterranean salt lens (Meddy) over a two year period is examined. Several nondimensional numbers can be used to describe the overall decay in the structure of the Moddy. Two Rossby numbers, one using the central relative vorticity and another using the radius and velocity of the azimuthal velocity maximum, did not change over the two year period. However, the Burger number N2H2/(f2L2) increased as the Meddy decayed. Another Burger number, the ratio of total kinetic energy to total available potential energy, decreased from 1.1 to 0.6 over a one year period. The rates at which the Meddy lost salt and heat are consistent with estimates of horizontal fluxes by intrusions. A horizontal diffusivity of O(5 m2 s−1) is needed if this flux by intrusions is parameterized by an eddy coefficient. Simple models of the evolution of an isolated eddy by horizontal and vertical mixing of mass and momentum are examined. These simple attempts to explain the evolution of the Meddy suggest more c...

Oceanic thermohaline intrusions: observations

Intrusions are commonly observed in the upper, deep and coastal oceans, and are closely linked to lateral fluxes of heat, salt and momentum. This is a review of observations of intrusions and the results of comparisons of properties such as scale, slopes, microstructure activity, and fluxes with theoretical models. A summary of estimates of lateral heat fluxes indicates a wide range of lateral diffusivities. We conclude by noting that our present knowledge is insufficient to predict the structure, length-scales and lateral fluxes of thermohaline intrusions with confidence, and list a number of unresolved questions. Suggestions are made for compilation of existing data into a database for exploratory analysis and testing of theoretical hypotheses. An outline is given of a potential collaborative field experiment using CTD, fluorescent dye, and microstructure observations.

The formation of layers in a uniformly stirred density gradient

Abstract Experiments are described in which an initially smooth density gradient breaks down into a regular series of steps and layers when it is stirred weakly by an array of vertical rods. When stirring is more vigorous, layers do not form, and irregularities in the density gradient tend to be smoothed out. The observed behaviour lends strong support to the hypothesis of Phillips (1972, Deep-Sea Research, 19, 79–81) and others, that if the buoyancy flux is a decreasing function of Richardson number, then an instability occurs, that leads to the formation of fine-structure from a smooth density gradient. Any local increase in density gradient inhibits the buoyancy flux at that level, so allowing the density gradient to steepen further. The scale of the layers that are formed, the variation of buoyancy flux (and, indirectly, mixing efficiency) with stirring rate, and the effects of intermittent stirring are also discussed.

Oceanic double-infusion: introduction

Abstract Double-diffusion, the mixing of fluids with two constituents of different molecular diffusivities, was originally discovered in the mid-1800s, forgotten, then rediscovered as an ‘oceanographic curiosity’ a century later. Many oceanographers suspect that double-diffusion has major effects on oceanic water masses and circulation, but direct measurement of the effects has proven difficult. In 1996, a Working Group was formed under the auspices of the Scientific Committee on Ocean Research (SCOR WG108), with the goal to identify progress and barriers to quantifying oceanic double-diffusive fluxes, and make recommendations for further progress. This document gives a brief history of double-diffusion, a review of evidence of its potential effects in the ocean, and gives an overview of the review articles contained in this volume, written by the Working Group members with the above aim in mind.

The use of ocean microstructure to quantify both turbulent mixing and salt-fingering

Abstract A method is developed whereby the vertical fluxes of heat and salt can be deduced from oceanic microstructure measurements of temperature and shear variance. This method is appropriate when both turbulent mixing and double-diffusive convection contribute to the vertical property fluxes. Previous methods that deduce property fluxes from microstructure measurements have assumed that either turbulent mixing or double-diffusive convection is the cause of the observed microstructure; here we present a method suitable for the more general situation where both mixing processes contribute to the property fluxes in the region of interest. The key assumptions are that the mixing efficiency of the turbulence and the flux ratio of the salt fingers are unchanged by the presence of the other process. It is found that sufficiently accurate total heat and salt fluxes can be deduced from the microstructure data, but our present imprecise knowledge of the salt-finger buoyancy flux ratio places large error bars on the total buoyancy flux.

The Mixing of Meddy “Sharon”

Abstract It is suggested that thermohaline intrusions are the dominant mechanism for mixing of a Mediterranean salt lens, and vertical and horizontal diffusivities for salt are estimated. Models of thermohaline intrusions are reviewed, and their predictions are compared to the lens observations. It was found that the “Meddy” front can be considered a wide front in the sense that the dynamics depend on the local value of the horizontal salinity gradient.