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M. C. Gregg

Abstract

The development of high-resolution profiling instruments has made possible the computation of quantities such as the stability parameter N 2 and the Richardson number Ri over scales of less than a meter. Quality control of the data has been done on an ad hoc basis by individual investigators who have used varying degrees of restraint in the claims made for the final results. As the amount of high-resolution data increases and as the parameters computed from it are used in statistical studies a more objective approach is required.

Profiting instruments are subject to a wide variety of difficulties, many of which are particular to the individual systems. Common to all, however, are errors due to uncertainties in the absolute calibrations of temperature T, conductivity C and pressure P, and noise in the sensors or data systems. Linearized equations for salinity S and specific volume α have been found to give good estimates of the errors resulting from typical values of calibration uncertainties and noise in T, C and P. Using these equations expressions have been developed for the evaluation of corresponding uncertainties and noise in N 2, Ri and dynamic height anomaly as well as in maps of T and S on potential specific volume surfaces. These estimates represent the best that can be obtained if the instrument system has no other difficulties. Numerical examples for several instrument systems in current use show that significant errors can occur over scales as large as tens of meters.

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M. C. Gregg

Abstract

Finestructure and microstructure observations of temperature and salinity were made over a two-week period in late winter at a site in the central North Pacific, during which time a mild storm, maximum winds of 18 kt, passed through the area. The strong initial lateral variability in T and S, but weak vertical stratification, was altered apparently by mixing due to surface cooling and the storm winds to produce several discrete types of water. These began to spread laterally several days after the onset of the storm and resulted in a triple step structure of 50 m thick homogeneous layers at one location.

Temperature fluctuations in the mixed layers varied between 10−3 and 10−2°C rms, and χ, the rate of destruction of temperature fluctuations, ranged from 1×10−10 to 4×10−2°C−2 s−1. The lower values occurred early in the storm after the mixed layer reached an equilibrium depth of about 100 m. The 2.5 m thick transition region at the base had a sharp interface, 0.25 m thick, but there was no evidence of entrainment. Apparently in the absence of a strong source of temperature fluctuations, T −2 had been reduced by the turbulence. A few days later a mixed layer half as thick, apparently resulting from the intrusion of denser water beneath, showed a transition with numerous small-scale instabilities and an overturn on a 0.12 m thick interface directly below the mixed layer. Spectra of the three observations during the storm showed fairly good fits to the “universal” spectral forms for temperature fluctuations in fully-developed turbulence, unlike the spectra of data taken after the storm.

At the one station that did not have an intrusion in the surface layer, the change in the heat content of the surface water, due to the air-sea exchange, was much less than that of the deeper water, which was replaced by on intrusion. Thus, when strong lateral variability exists, e.g., in late winter off the main storm track, the sinking and lateral spreading motions of the type proposed by Stommel and Fedorov (1967) resulting from even a mild storm, may be much more important than the direct vertical mixing.

In the stratified water below the surface layers the levels of microstructure detected during the storm appeared little different from those found afterward.

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M. C. Gregg

Abstract

Two microstructure records taken at shallow depths off Cabo San Lucas, at the southern tip of Baja California, are compared. One is similar to records previously taken in the mid-gyre, and has an “irregularly steppy” appearance, a linear T-S relation, and a Cox number of approximately 10. It is suggested that this type of profile may represent the background condition of the ocean in which the levels of vertical turbulence are quite low and the principal dissipation occurs by small-scale shear instabilities at the “step“ structures. The other record exhibits a very irregular T-S relation, due to multiple interleavings of the water masses present in the area. Coupled with this is an average Cox number of at least 6000 and a much greater variability in the local microstructure levels along the record; half-meter averages of the dissipation rate of temperature fluctuations show a range greater than 106. In some cases these differences occur over vertical separations of a few meters. In general, the region of intense microstructure activity occur at the vertical boundaries of the intrusions and seem to be the result of shears and double diffuse phenomena associated with the spreading motion of the intrusion and the vertical T-S differences. These processes act to dissipate the intrusion as an identifiable feature. Off the California coast the 5 to 30 m thick intrusions which are associated with strong microstructure are produced by the interleaving of northward flowing warm saline Equatorial Water and the southward moving cool fresh California Current. The amplitude and frequency of the intrusions decrease at stations closer to the subtropical gyre.

The presence of similar intrusive features in other locations suggests that they are major factors in the dissipation of fluctuations in the ocean, but microstructure profiles, by themselves, are not sufficient to assess the vertical heat flux associated with them.

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M. C. Gregg

Abstract

Extending the range of microstructure measurements from the upper thermocline of the open ocean to shallow waters near shore and to the abyss has greatly increased in the range of turbulent intensities being observed. As a result, it is necessary to reexamine the adequacy of microstructure probes to resolve dissipation-scale gradients of velocity and temperature. These variances are needed to estimate directly the viscous dissipation rate, ε, with airfoil probes and the diffusive dissipation rate, χ T, with thermistors.

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M. C. Gregg

Abstract

An expression that includes salt diffusion was developed and evaluated for the rate of entropy production σ in the ocean. The evaluation was done for mixing events produced by turbulent overturns and double diffusion. Temperature and salinity profiles from four representative oceanic regimes were used in the evaluation. For typical mid- or low-latitude profiles thermal diffusion is the main component of σ in upper-ocean turbulent events; in abyssal waters, viscous dissipation is the principal component. The depth of transition from dominance by σt to dominance by σv depends upon the depth dependence of mixing intensity, which has not been determined. Salt diffusion plays a major role only in a few locations where salinity strongly dominates the density field; the most prominent example is the shallow arctic halocline. Elsewhere, the effect of salt diffusion, However, when the viscous dissipation within the well-mixed layers above and below salt-fingering interfaces is also considered, viscous dissipation may be the dominant factor. The possible effect of viscous dissipation and the heat of mixing in the heat equation is also considered and found to be negligible unless strong turbulent dissipation rates are found in the bottom boundary layer, or intense mixing events occur in arctic haloclines.

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M. C. Gregg

Abstract

Large numbers of STD casts taken during five cruises to the same location in the central North Pacific permitted the separation of the temperature profile into mean T¯ (z) and fluctuating T′(z) components. Spectra of the T′ (z) records were ensemble-averaged by cruise and compared with similar spectra from one cruise to the South Pacific and with those from the MODE site. Two wavenumber ranges with power law behavior were found: for 0.002 cpm <k<(0.06−0.1) cpm the spectra fall with slopes slightly less than k −2, for 0.1 cpm<k<2 cpm the spectra fall as k −3. Spectra in the low k range were consistent with formation of T′(z) by internal waves. The break in slope at 0.1 cpm suggests that either the internal wave dynamics change at that scale or the smaller fluctuations are true finestructure. The spectral levels remained the same at the North Pacific location suggesting that the internal wave energy levels are stationary in time at that location. However, there was a factor of 10 variation of the levels at the three different sites. The vertical displacement of an isotherm about its mean depth was found to have a normal distribution, consistent with the assumption that internal waves are a Gaussian process. The average number of zero crossings of the displacement between 200 and 712 m varied between 3.5 to 5.

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M. C. Gregg

Abstract

Profiles of temperature microstructure have been found to exhibit strong intermittence in the distribution of χ, the rate of diffusive smoothing of temperature fluctuations. Records from the main thermocline in the subtropical gyro of the Pacific have been examined to determine the thickness of patches of microstructure activity. Since these profiles have few thermohaline intrusions, the small-scale structure must be generated by the “breaking” of internal waves or by salt fingering. The variations in the intensity of the microstructure thus reflect the irregularity in time and space of the occurrence of these processes. In addition, there is an intermittence of the microstructure due to variations in intensity of the microstructure within individual mixing events. Much of this internal variation is due to modulation of the amplitude of the microscale temperature fluctuations by the larger scale finestructure.

In an attempt to compensate for the second type of intermittence, zero-crossings between positive and negative gradients were used as indicators of activity. The regions with zero-crossings account for most of the activity in those records having large Cox numbers. Individual negative gradients appear to be static instabilities that are no more than 5 cm thick, giving maximum lifetimes much less than N−1 if the temperature structures are no longer accompanied by velocity fluctuations over corresponding scales. Hence the patches are active and are not the decayed fossils of previous events, The fractions of the profiles containing activity varied from 7 to 36%, and may be underestimates by a factor of 2. While many of the patches were 2 m or less in thickness, some that appeared continuously active for 30 m were found. The regions containing zero-crossings were distributed independently of the finestructure.

The patterns of microstructure are not consistent with those expected if salt fingering is the dominant process in the upper thermocline, which is diffusively unstable. Neither, in most cases, are they indicative of Kelvin-Helmholtz instabilities occurring preferentially in high-gradient regions. The relatively short scales of possible overturns in comparison with the thickness of many of the patches suggest that the thicker patches may be due to a sequence of vertical overturns rather than one large overturn.

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M. C. Gregg

Abstract

Three cruises to the same site in the central North Pacific (28°N, 155°W) found naked differences in the levels of small-scale turbulent activity as indicated by spectra of temperature profiles. Considering the data from all depths the mean values of the normalized variance of the temperature gradients, or Cox number, were 2 (September 1971), 10 (June 1973) and 59 (February 1974). Data taken during winter at a corresponding site in the South Pacific (28°S, 155°W) were similar to the latter values. The most active records were found below the main thermocline or in the seasonal thermocline and exhibited gradient spectra with slopes between +½ and +1 from 2 cpm to greater than 90 cpm. The less active records were diffusively cut off for k>10 cpm. The level of microstructure activity was not related to the spectral levels at lower wavenumber. The variance of the more active records was contributed by several intense patches a few meters thick, which had ranges of greater than a decade in the gradient spectra with spectral slopes of +½ to +1. One of the patches contained a 1.3 m density instability similar to overturning vortex structures studied in laboratory experiments. Even in the active patches the spectra did not resemble the forms seen in fully developed homogeneous turbulence. A few records taken at abyssal depths were not markedly different from the moderately active records in the thermocline.

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Yves Desaubies and M. C. Gregg

Abstract

Various statistics of temperature profiles are examined in an attempt to distinguish irreversible structures due to mixing from reversible distortions induced by internal wave straining. Even if all the low gradient regions were the result of mixing events, an analysis of the profiles shows that such events are rare and most often incomplete. An upper bound on the mixing effectiveness is obtained; it increases as the vertical scale decreases. Taking next the opposite view that internal wave straining is the sole process, an analytic model is developed to calculate the probability density function of temperature gradients. The model considers the straining by a weakly nonlinear Gaussian internal wave field of a linear temperature profile. The nonlinearity of the field is essential to account for the skewness of the probability distributions. Comparisons with data are quite satisfactory at scales larger than ∼2 m, less so at smaller scales. We conclude that nonlinear effects are important; at scales larger than ∼2 m straining is dominant with very little mixing, while at smaller scales irreversible structures are more prevalent.

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M. C. Gregg and T. B. Sanford

Abstract

No abstract available.

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