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Dave Hebert

Abstract

The driving mechanism for the observed interleaving of water masses is generally assumed to be double-diffusive mixing. However, some observations of intrusions have been made in regions where the mean stratification is stable to double-diffusive mixing. It has been hypothesized that a finite amplitude disturbance must occur to produce regions where the stratification allows double-diffusive mixing or that an instability due to differences in the molecular diffusivity of salinity and temperature produces the desired stratification for double-diffusive mixing to start. There is also the possibility of a differential vertical flux of salt and heat due to incomplete mixing by turbulence. The basis of this idea is described in this paper. Growth rates, vertical scales, and cross-frontal slopes of intrusions predicted by this process are compared to those predicted by double-diffusive mixing.

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Dave Hebert

Abstract

The variation in the herizontal Reynolds stress due to the internal wave field with respect to the location of the Gulf Stream is examined. These measurements are from an array of current meter moorings placed in the mean path of the Gulf Stream at 59°W. Temperature was used as an indication of the current meter's position relative to the Gulf Stream. No systematic variation in the Reynolds stress with respect to the location of the Gulf Stream can be seen. From estimates of the large-scale Horizontal shear, an upper limit for the magnitude of the horizontal eddy viscosity coefficient was found to be 50 m2 s−1.

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Chris Roman
and
Dave Hebert

Abstract

Efficiently profiling the water column to achieve both high vertical and horizontal resolution from a moving vessel in deep water is difficult. Current solutions, such as CTD tow-yos, moving vessel profilers, and undulating tow bodies, are limited by ship speed or water depth. As a consequence, it is difficult to obtain oceanographic sections with sufficient resolution to identify many relevant scales over the deeper sections of the water column. This paper presents a new concept for a profiling vehicle that slides up and down a towed wire in a controlled manner using the lift created by wing foils. The wings provide a novel low-power method of propulsion along the cable by using the free stream velocity of the wire moving through the water in similar fashion to a sailboat sailing up wind. Scale model tests show a wide range of achievable profiling glide slopes for tow cable angles between vertical and 45°, and effective isolation of cable strum vibration from the towed vehicle body. The concept is not depth limited and will offer two-dimensional resolution that meets or exceeds current undulating tow bodies over the full water column. Additionally, this system could be used simultaneously with many other deep towed instrument packages to produce complementary datasets.

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Dave Hebert
and
J. N. Moum

Abstract

The decay of a downward propagating near-inertial wave was observed over four days. During this short period, the energy of the near-inertial wave decreased by 70%. The shear layers produced by the wave were regions of enhanced turbulent dissipation rates. The authors estimate that 44% of the observed change in the near-inertial energy was lost to turbulence. Estimates of the wave energy lost at the survey site due to the wave propagating out of the region were smaller. Energy lost by horizontal advection of the wave out of the survey region was more difficult to estimate; the horizontal extent of the near-inertial energy was unknown. Advection could account for more than half of the observed energy lost. However, the authors did not detect the near-inertial wave during a 40 km×40 km ADCP survey after completing the six-day station.

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Dave Hebert
,
Neil Oakey
, and
Barry Ruddick

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 N 2 H 2/(f 2 L 2) 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 complicated models are necessary to reproduce its decay.

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Grant A. Stuart
,
Miles A. Sundermeyer
, and
Dave Hebert

Abstract

Geostrophic adjustment of an isolated axisymmetric lens was examined to better understand the dependence of radial displacements and the adjusted velocity on the Burger number and the geometry of initial conditions. The behavior of the adjustment was examined using laboratory experiments and numerical simulations, which were in turn compared to published analytical solutions. Three defining length scales of the initial conditions were used to distinguish between various asymptotic behaviors for large and small Burger numbers: the Rossby radius of deformation, the horizontal length scale of the initial density defect, and the horizontal length scale of the initial pressure gradient. Numerical simulations for the fully nonlinear time-dependent adjustment agreed both qualitatively and quantitatively with analogous analytical solutions. For large Burger numbers, similar agreement was found in laboratory experiments. Results show that a broad range of final states can result from different initial geometries, depending on the values of the relevant length scales and the Burger number computed from initial conditions. For Burger numbers much larger or smaller than unity, differences between different initial geometries can readily exceed an order of magnitude for both displacement and velocity.

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Chris Roman
,
David S. Ullman
,
Dave Hebert
, and
Stephen Licht

Abstract

The Wire Flyer towed vehicle is a new platform able to collect high-resolution water column sections. The vehicle is motivated by a desire to effectively capture spatial structures at the submesoscale. The vehicle fills a niche that is not achieved by other existing towed and repeat profiling systems. The Wire Flyer profiles up and down along a ship-towed cable autonomously using controllable wings for propulsion. At ship speeds between 2 and 5 kt (1.02–2.55 m s−1), the vehicle is able to profile over prescribed depth bands down to 1000 m. The vehicle carries sensors for conductivity, temperature, depth, oxygen, turbidity, chlorophyll, pH, and oxidation reduction potential. During normal operations the vehicle is typically commanded to cover vertical regions between 300 and 400 m in height with profiles that repeat at kilometer spacing. The vertical profiling speed can be user specified up to 150 m min−1. The high-density sampling capability at depths below the upper few hundred meters makes the vehicle distinct from other systems. During operations an acoustic modem is used to communicate with the vehicle to provide status information, data samples, and the ability to modify the sampling pattern. This paper provides an overview of the vehicle system, describes its operation, and presents results from several cruises.

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John A. Barth
,
Dave Hebert
,
Andrew C. Dale
, and
David S. Ullman

Abstract

By mapping the three-dimensional density field while simultaneously tracking a subsurface, isopycnal float, direct observations of upwelling along a shelfbreak front were made on the southern flank of Georges Bank. The thermohaline and bio-optical fields were mapped using a towed undulating vehicle, and horizontal velocity was measured with a shipboard acoustic Doppler current profiler. A subsurface isopycnal float capable of measuring diapycnal flow past the float was acoustically tracked from the ship. The float was released near the foot of the shelfbreak front (95–100-m isobath) and moved 15 km seaward as it rose from 80 to 50 m along the sloping frontal isopycnals over a 2-day deployment. The float's average westward velocity was 0.09 m s−1, while a drifter drogued at 15 m released at the same location moved westward essentially alongfront at 0.18 m s−1. The float measured strong downward vertical velocities (in excess of 0.02 m s−1) associated with propagation of internal tidal solibores in the onbank direction from their formation near the shelf break. The float measured large upward vertical velocities (in excess of 0.001 m s−1 ≃ 100 m day−1) as the pycnocline rebounded adiabatically after the passage of the internal tide solibore. The directly measured mean along-isopycnal vertical velocity was 17.5 m day−1. Intense mixing events lasting up to 2 hours were observed in the shelfbreak front at the boundary between cold, fresh shelf water and warm, salty slope water. Diapycnal velocities of up to 3 × 10−3 m s−1 were measured, implying a diapycnal thermal diffusivity as large as 10−2 m2 s−1, indicative of strong mixing events in this coastal front.

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Laurence Armi
,
Dave Hebert
,
Neil Oakey
,
James F. Price
,
Philip L. Richardson
,
H. Thomas Rossby
, and
Barry Ruddick

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 be the most important mode of mixing in terms of salt and heat loss.

Radial profiles of azimuthal velocity revealed a core in almost solid body rotation, with a period of 5–6 days corresponding to 0.35 times the local Coriolis parameter. During the October 1984 survey, the azimuthal speed had a maximum of 0.3 m s−1 at a radius of 24 km. Both the radius and magnitude of the velocity maximum decreased with time. The anticyclonic circulation attained a maximum at the radius of the salinity front. As the lens was eroded from the sides, the radius of maximum velocity and the maximum velocity both decreased, but the rotation rate of the core remained fairly steady.

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