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J. N. Moum

Abstract

A method is described for measuring the vertical component of velocity fluctuations due to three-dimensional turbulence in the ocean from a freely falling microstructure profiler. The dynamic pressure measurement relies on a commercially available and very sensitive piezoresistive differential pressure transducer. At nominal profiler fall speeds of 0.9 m s−1, the noise limit in vertical velocity fluctuations is ∼3 mm s−1. Scaled turbulence spectra from shear probes and from the new sensor demonstrate the validity of the measurement. Although hydrodynamic noise (eddy-shedding) at frequencies near the peak in the dissipation spectrum precludes full resolution of the high wavenumber portion of the spectrum, the high end of the inertial subrange of the turbulence is resolved. At low wavenumbers, the measurement is limited by the finite size of the profiler. It is anticipated that the method will be useful in examining turbulent fluxes using an eddy-correlation technique.

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A. Anis and J. N. Moum

Abstract

A freely rising profiler was used to collect vertical microstructure profiles in the upper oceanic boundary layer under various atmospheric and sea conditions. Near the sea surface, the rate of viscous dissipation of turbulence kinetic energy, ε, exhibited a range of behaviors under different forcing conditions. Sometimes, ε was closely balanced by the wind stress production of turbulence kinetic energy. At other times, ε was greatly enhanced relative to wind stress production and exhibited an exponential depth deny. In these instances, simple scaling laws predicted for turbulence near a solid surface severely underestimate turbulent mixing near the ocean surface.

Plausible explanations for enhanced ε(z) near the sea surface will have to address the effects of wave-turbulence interactions. The authors propose two different mechanisms to explain the behavior of ε near the surface, leading to two scaling schemes. The first mechanism requires high levels of turbulence kinetic energy, created by wave breaking at the surface, to be transported downward away from the surface by the motion of the swell. This transport is then locally balanced by ε. The second mechanism requires a rotational wave field and significant wave stresses that balance the turbulence Reynolds stresses. Energy drawn from the wave field to the mean flow, via the wave stresses, is in turn drawn from the mean flow by the turbulence production term, which is balanced by ε.

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A. Perlin and J. N. Moum

Abstract

As a quantitative test of moored mixing measurements using χpods, a comparison experiment was conducted at 0°, 140°W in October–November 2008. The following three measurement elements were involved: (i) NOAA’s Tropical Atmosphere Ocean (TAO) mooring with five χpods, (ii) a similar mooring 9 km away with seven χpods, and (iii) Chameleon turbulence profiles at an intermediate location.

Dissipation rates of temperature variance and turbulent kinetic energy are compared. In all but 3 of 17 direct comparisons 15-day mean values of χT agreed within 95% bootstrap confidence limits computed with the conservative assumption that individual 1-min χpod averages and individual Chameleon profiles are independent. However, significant mean differences occur on 2-day averages. Averaging in time reduces the range (95%) in the observed differences at two locations from a factor of 17 at 1-day averaging time to less than a factor of 2 at 15 days, presumably reflecting the natural variability in both the turbulence and the small-scale fluid dynamics that lead to instability and turbulence.

The motion of χpod on a mooring beneath a surface buoy is complex and requires a complete motion package to define in detail. However, perfect knowledge of the motion of the sensor tip is not necessary to obtain a reasonable measure of χT. A sampling test indicated that the most important motion sensor is a pressure sensor sampled rapidly enough to resolve the surface wave–induced motion.

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A. Anis and J. N. Moum

Abstract

A detailed investigation of the upper ocean during convection reveals

  1. • the vertical structure of potential temperature, θ, to be steady in time, and
  2. • the current shear to vanish in the bulk of the mixed layer.
These imply that a “slab”-type model may be an adequate representation of the convective ocean boundary layer (OBL). In contrast, when convection is not the dominant forcing mechanism, the OBL is stratified and can support a significant current shear. This indicates the inadequacy of “slab” models for the nonconvective OBL.

Two independent estimates of the vertical heat flux profile in the convective OBL were made. The first estimate results from heat conservation and the steadiness of the vertical structure of potential temperature. The second estimate is based on the turbulent kinetic energy (TKE) balance and the vertical profiles of TKE dissipation rate. The estimates are consistent and suggest that the nondimensional vertical heat flux due to turbulence has a linear depth dependence of the form 1 + ah(z/D), where z is the depth, D is the mixed layer depth, and ah is a constant with a mean value of 1.13, consistent with numerical and laboratory results and with observations in the convective atmospheric boundary layer. An estimate of the entrainment rate, derived from observed quantities, is ∼1 × 10−5 m s−1. This is within a factor of 2 of estimates derived from alternative formulations.

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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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J. N. Moum and J. D. Nash

Abstract

Highly resolved pressure measurements on the seafloor over New Jersey’s continental shelf reveal the pressure signature of nonlinear internal waves of depression as negative pressure perturbations. The sign of the perturbation is determined by the dominance of the internal hydrostatic pressure (p 0 Wh) due to isopycnal displacement over the contributions of external hydrostatic pressure (ρ 0gηH; ηH is surface displacement) and nonhydrostatic pressure (p 0 nh), each of opposite sign to p 0 Wh. This measurement represents experimental confirmation of the wave-induced pressure signal inferred in a previous study by Moum and Smyth.

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J. N. Moum and J. D. Nash

Abstract

Recent turbulence measurements over a small bank on the continental shelf off Oregon reveal a previously undetected site for intense mixing of the coastal ocean. The flow is hydraulically controlled and turbulence diffusivities over the bank are more than 100 times greater than estimates made on the shelf away from topography. The total drag exerted by the bank on the flow field is a combination of bottom friction plus form drag (analogous to mountain drag) and is comparable to the Coriolis force. This drag is sufficient to decelerate the flow over the bank in a matter of hours.

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J. N. Moum and T. R. Osborn

Abstract

A series of profiles of velocity microstructure along 152°E in the western North Pacific Ocean were collected in May–June 1982. Large, averaged turbulent dissipation rates, ε, found in the main thermocline (400 to 1000 m) were determined by a combination of large independent estimates of ε and a greater rate of occurrence of turbulent events in the main thermocline than elsewhere. Concurrently we find that averaged values of ε exhibit a positive dependence on the buoyancy frequency, N, and that form ε = aN γ is best fit by γ = 1 when only the data below 400 m are considered. Of the more than 5000 m of data collected below 1000 m depth, 12% showed measurable turbulence and dominated the depth averages. A deep ocean estimate of an upper bound to the eddy coefficient for vertical diffusion, K ρ, is 10−4 m2 s−1 and not significantly different from the value estimated by Munk. The inferred dependence of the mass flux with depth indicates the relative significance of vertical mixing in the main thermocline. Other processes must influence the maintenance of the more weakly stratified 15°–18°C water above.

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Ann E. Gargett and J. N. Moum

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

  1. • Γd is no different from that determined in other open ocean experiments and is independent of the sign of the flux
  2. • Γ d < |Γo|, regardless of the sign of the flux
  3. • Γo (flux > 0) < |Γo| (flux < 0).

The consequences for interpretation of ocean microstructure flux estimates are discussed.

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J. N. Moum, J. D. Nash, and W. D. Smyth

Abstract

Extended measurements of temperature fluctuations that include the turbulence wavenumber band have now been made using rapidly sampled fast thermistors at multiple depths above the core of the Equatorial Undercurrent on the Tropical Atmosphere Ocean (TAO) mooring at 0°, 140°W. These measurements include the signature of narrowband oscillations as well as turbulence, from which the temperature variance dissipation rate χT and the turbulence energy dissipation rate ϵχ are estimated.

The narrowband oscillations are characterized by the following:

  • groupiness—packets consist of O(10) oscillations;
  • spectral peaks of up to two orders of magnitude above background;
  • a clear day–night cycle with more intensive activity at night;
  • enhanced mixing rates;
  • frequencies of 1–2 × 10−3 Hz, close to both the local buoyancy and shear frequencies, N/2π and S/2π, which vary slowly in time;
  • high vertical coherence over at least 30-m scales; and
  • abrupt vertical phase change (π/2 over <20 m).
The abrupt vertical phase change is consistent with instabilities formed in stratified shear flows. Linear stability analysis applied to measured velocity and density profiles leads to predicted frequencies that match those of the observed oscillations. This correspondence suggests that the observed oscillation frequencies are set by the phase speed and wavelength of instabilities as opposed to the Doppler shifting of internal gravity waves with intrinsic frequency set by the local stratification N.

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