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

Abstract

Direct numerical simulations are used to compare turbulent diffusivities of heat and salt during the growth and collapse of Kelvin–Helmholtz billows. The ratio of diffusivities is obtained as a function of buoyancy Reynolds number Reb and of the density ratio Rρ (the ratio of the contributions of heat and salt to the density stratification). The diffusivity ratio is generally less than unity (heat is mixed more effectively than salt), but it approaches unity with increasing Reb and also with increasing Rρ. Instantaneous diffusivity ratios near unity are achieved during the most turbulent phase of the event even when Reb is small; much of the Reb dependence results from the fact that, at higher Reb, the diffusivity ratio remains close to unity for a longer time after the turbulence decays. An explanation for this is proposed in terms of the Batchelor scaling for scalar fields. Results are interpreted in terms of the dynamics of turbulent Kelvin–Helmholtz billows, and are compared in detail with previous studies of differential diffusion in numerical, laboratory, and observational contexts. The overall picture suggests that the diffusivities become approximately equal when Reb exceeds O(102). The effect of Rρ is significant only when Reb is less than this value.

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

Abstract

Narrowband oscillations observed in the upper equatorial Pacific are interpreted in terms of a random ensemble of shear instability events. Linear perturbation analysis is applied to hourly averaged profiles of velocity and density over a 54-day interval, yielding a total of 337 unstable modes. Composite profiles of mean states and eigenfunctions surrounding the critical levels suggest that the standard hyperbolic tangent model of Kelvin–Helmholtz (KH) instability is a reasonable approximation, but the symmetry of the composite perturbation is broken by the stratification and vorticity gradient of the underlying equatorial undercurrent. Unstable modes are found to occupy a range of frequencies with a peak near 1.4 mHz, consistent with the frequency content of the observed oscillations.

A probabilistic theory of random instabilities predicts this peak frequency closely. An order of magnitude estimate suggests that the peak frequency is of order N, in accord with the observations. This results not from gravity wave physics but from the balance of shear and stratification that governs shear instability in geophysical flows. More generally, it is concluded that oscillatory signals with frequency bounded by N can result from a process that has nothing to do with gravity waves.

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E. L. Shroyer, J. N. Moum, and J. D. Nash

Abstract

Observations off the New Jersey coast document the shoaling of three groups of nonlinear internal waves of depression over 35 km across the shelf. Each wave group experienced changing background conditions along its shoreward transit. Despite different wave environments, a clear pattern emerges. Nearly symmetric waves propagating into shallow water develop an asymmetric shape; in the wave reference frame, the leading edge accelerates causing the front face to broaden while the trailing face remains steep. This trend continues until the front edge and face of the leading depression wave become unidentifiable and a near-bottom elevation wave emerges, formed from the trailing face of the initial depression wave and the leading face of the following wave. The transition from depression to elevation waves is diagnosed by the integrated wave vorticity, which changes sign as the wave’s polarity changes sign. This transition is predicted by the sign change of the coefficient of the nonlinear term in the KdV equation, when evaluated using observed profiles of stratification and velocity.

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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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James N. Moum, Douglas R. Caldwell, and Phyllis J. Stabeno

Abstract

During August 1986, a large cold anomaly was observed in satellite and in situ measurements near Cape Blanco at 42°N, 126°30′W off the Pacific Coast. Detailed vertical profiles of temperature, conductivity, turbulent dissipation, and horizontal currents showed 1) surface water temperature changes as large as 2 degrees in 1 kilometer (but smaller gradients at depth); 2) a structure in the mean currents resembling that of either a cyclonic eddy or a current meander, 3) a current field in geostrophic balance on scales of 10 km and greater, 4) a region of intrusions on the northern side of the eddy; 5) a concentration of turbulence (as indicated by the kinetic-energy dissipation rate) on the edges of the eddy and in the region of intrusions, the core of the eddy being turbulence-free; and 6) a substantial change in the surface structure in 24 hours.

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J. N. Moum, T. R. Osborn, and W. R. Crawford

Abstract

Turbulence measurements from the central equatorial Pacific in February 1982 have been analyzed and compared to synoptic CTD and current velocity profiles and current meter data. These suggest considerably more time (if not space) variability than had previously been anticipated. Above 300 m at the equator the turbulence levels were greater but less than previous equatorial measurements, and turbulent patches occurred more frequently than elsewhere in the open ocean. Below 300 m the occurrence of turbulent patches was less frequent than in other regions of the ocean, except for the persistence of a patch at 500 m.

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W. D. Smyth, P. O. Zavialov, and J. N. Moum

Abstract

Measurements of velocity, hydrography, surface meteorology, and microstructure were made through several squall events during a westerly wind burst that occurred in the Western Pacific warm pool in December 1992. Sustained wind forcing generated a weakly stratified turbulent surface layer that extended to the top of the main thermocline. Following each rain event, freshwater formed a statically stable layer in the upper 4–12 m. The subsequent evolution of the mixing profile was strongly depth-dependent. Turbulence increased dramatically in the fresh layer adjacent to the surface but decreased in the underlying layer. The factor by which turbulence decreased following a given squall was strongly correlated with the net rainfall. The observed timescale for the decay of the turbulence was about 0.7 buoyancy periods, similar to decay times observed near the surface after sunrise. However, these decay times are significantly larger than those estimated indirectly (as the ratio of dissipation rate to turbulent kinetic energy) from turbulent patches in the thermocline. To account for the discrepancy, the authors hypothesize that turbulence production continues to act during the observed decay process, partially counteracting the effect of dissipation.

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D. Hebert, J. N. Moum, and D. R. Caldwell

Abstract

In spite of the effects of several form of temporal variability that tend to mask geographical patterns in turbulence intensity, our evidence indicates that the turbulence is enhanced above the equatorial undercurrent in comparison to latitudes north and south of it. This evidence consists of three meridional transects of micro-structure observations across the equator (at 140°W in 1984 and 1987. and at 110°W in 1987) along with an equatorial station at 140°W and a longitudinal transect along the equator from 140°W to 110°W. All three meridional transects show a peak in averaged estimates of the turbulent kinetic energy dissipation rates, ε, at the equator, although in 1984 the peak was not significant at the 95% level. The major sources a temporal variability were the diurnal buoyancy flux variation and the wind stress variations, which had a typical period of a few days. After the diurnal variability is removed by averaging, it can be shown that, for similar wind stress, ε is larger over the undercurrent than away from it. Examination of the 16-m, 1-hour averaged ε, in terms of the vertical shear of horizontal velocity and the stratification (determined over similar space and time scales), indicated a tendency of this mean ε to vary with the Richardson number, Ri, when Ri<1. However, closer examination showed that the dependence of ε on Ri varied with depth. Therefore, a simple parameterization for mixing rates on Ri is not valid for all depth.

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