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B. A. Elliott and T. B. Sanford

Abstract

The dynamics of a subthermocline lens observed during the POLYMODE Local Dynamics Experiment are examined using density data and measurements of the velocity field obtained by an absolute velocity profiler. It is shown that the momentum balance is nonlinear. The lens' potential vorticity contours are closed in the horizontal and vertical, trapping low-salinity water at the lens core. The lens's dynamics are explained by a series of elementary models based on the classical Bessel-tunction vortex. The models show that nonlinearity enters in two ways, through the nonlinear momentum balance and through the finite character of the stretching vorticity. The models suggest a lens anatomy: the core; a boundary layer at the velocity maximum; a buffer zone; and a geostrophic region. The first two terms are self-explanatory. The buffer zone extends from the velocity maximum to a radius we term the geostrophic radius; at which there is a salinity front. On either side of this front the character of the mixing processes is quite different. At larger radii the momentum balance is geostrophic, and the lens remains a coherent structure through finite stretching vorticity.

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B. A. Elliott and T. B. Sanford

Abstract

One remarkable result of the Local Dynamics Experiment in POLYMODE is the discovery of a large number of small energetic eddies. Using CTD data, SOFAR float tracks, and profiles of absolute velocity, we describe one of these features—the subthermocline eddy D1. The feature has an advective time scale of 1 day, and its primary mode of movement is advection by the ambient flow field. The eddy is resolved into a coordinate system moving with the velocity of the ambient flow, which ranges from 11–16 cm s−1. Its phase speed is less than 2 cm s−1.

The eddy is a lens centered at 1500 db. Velocity profiles show the lens has no detectable signal in and above the main thermocline or below about 3000 db. Its subsurface velocity maximum is 28.6 cm s−1 at 1500 db, 15 km from the center. The radial variation of the azimuthal velocity is Gaussian inside the velocity maximum but decays as e br beyond. There is little evidence of the eddy beyond a radius of 25 km.

Inside the velocity maximum, the eddy is characterized by a salinity minimum of 34.965‰ over a potential temperature range of 3.838°–3.917°C and an oxygen maximum of 6.1 ml l−1. The freshest water is located in the lower half of the eddy about 10 km from the dynamic axis. However, the eddy is uniformly fresher than the ambient water over most of its density range.

The water property data on any density surface show radial and azimuthal structure. Well mixed, 50–100-db thick layers may indicate active mixing in the interior. Hence this eddy may have at one time been colder and fresher than observed here. Outside the velocity maximum to a radius of 25 km the water is a mixture of eddy water and the surrounding water. Based on water properties, the eddy core is predominantly of Labrador Sea origin.

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M. C. Gregg and T. B. Sanford

Abstract

No abstract available.

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D. Y. Lai and T. B. Sanford

Abstract

Velocity Profiles and current meter measurements taken near Site D(39°10′N, 70°00′W) on the continental rise south of New England are used to study the variability of the near-inertial wave field along a sloping bottom. While the typical vertical scales of the waves are on the order of 100 m, some energetic downward propagating near-inertial features are observed with unusually large vertical scales, on the order of the ocean depth. Comparison with an internal wave model on a linear bottom slope shows that these energetic waves are dominated by the lowest three downward and seaward printing dynamical “slope” modes. The lowest mode arrives first at Site D from the north, the higher modes follow several days later.

The observed scales and propagation directions suggest that the energetic near-inertial waves were generated by a hurricane and then reflected at the steep continental slope to the north of Site D. The low-order, flat-bottom modes that usually dominate the far-field response of a hurricane are changed by the sloping bottom into “slope” modes, which then propagate toward deeper water. Energy intensification of these modes towards the bottom suggests that sloping bottoms may play a significant role in the near-inertial wave field below the main thermocline.

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M. C. Gregg and T. B. Sanford

Abstract

Nearly simultaneous profiles of temperature microstructure and velocity shear were made adjacent to the island of Bermuda. Profiles with elevated microstructure levels were found in close association with regions of pronounced steplike finestructure, which contained nearly adiabatic regions from 2 to 10 m thick. The temperature spectra and the presence of numerous centimeter-scale temperature inversions gave evidence that active turbulent mixing was occurring in some of these regions. These were the locations in which large-scale surveys, reported by Hogg et al. (1978), found that eddies impinging on the island were forcing alongshore flow.

Although the mixing was intense by comparison with profiles in the thermocline, the limited geographical extent of the affected areas and the moderate levels indicate that mixing adjacent to islands is of minor importance on a global basis.

A very limited number of profiles taken in the Sargasso Sea found microstructure levels in the thermocline that were similar to previous data from the Pacific. In both sets of observations the microstructure levels are consistent with Kz levels significantly below 10−4 m2 s−1. Although the surface winds were very light, a 135 m deep mixed layer was turbulent. The spectral forms showed general, but not exact, agreement with the “universal” spectral forms.

The microstructure activity in the Gulf Stream was dominated by double-diffusive signatures on the upper and lower boundaries of the numerous thermohaline intrusions that were present. Thus the high shear values, 10−2 s−1, did not inhibit the formation of double-diffusive structures. In intervals not containing inversions, the microstructure levels were little different from those in the Sargasso Sea. These levels are much lower than those found in the Equatorial Undercurrent and are not consistent with the values assumed for vertical turbulent diffusivities in models of the Stream.

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A. E. Gargett, T. B. Sanford, and T. R. Osborn

Abstract

Observations of turbulent energy dissipation rate ε in the deep surface mixed layer at a mid-Sargasso site are presented: two occupations of this site include a large range of local meteorological forcing. Two frontal passages and a large time interval between profiles during the first series of measurements preclude examination of the turbulent kinetic energy balance: qualitatively, a profile taken during the strongest wind-wave forcing of the observation set suggests that layer deepening was not being driven directly from the surface, but by a shear instability at the mixed layer base. A quantitative assessment of terms in the steady-state locally balanced model of the turbulent kinetic energy budget proposed by Niiler (1975) has been possible for two profiles having dissipation characteristics and surface meteorological conditions which allow us to argue for the absence of all but a few of the possible source/sink terms in the turbulent kinetic energy balance. In one case, a steady-state local balance is possible. In the other case, a local balance can be maintained by giving up the steady-state assumption. i.e., by including the time rate of decay of the turbulent kinetic energy. Other possible balances exist. The analysis of the surface mixed-layer turbulent kinetic energy balance highlights two major uncertainties-parameterization of the wind-wave forcing term and lack of reliable dissipation measurements in the upper 10–20 m of the water column.

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R. H. Käse, H-H. Hinrichsen, and T. B. Sanford

Abstract

A method is presented for determining salinity and density from temperature data in conjunction with historical or contemporaneous (but not collocated) CTD observations. The horizontal density ratio r(z) is determined from the temperature and salinity differences at each depth (δT, δS) between pairs or ensembles of profiles. These differences are expressed as a density ratio r=αδT/βδS, where α and β are the expansion coefficients for temperature and salinity, respectively. Salinity at a site where only temperature is measured, as with an expendable bathythermograph (XBT), is computed based on the temperature and salinity at a reference station (S R,T R); that is, S=S R+(TT RST. The method is restrictive in its application because it is most accurate when all water masses in the region of a survey are linear extrapolations from the water masses at each of the reference stations. In reality, it provides useful results when the T and S fields are not simply linear functions of horizontal distance. This approach is particularly useful in regions where, the T(z)−S(z) relation is nonunique, as in the Mediterranean Water in the North Atlantic. The corresponding expression for the lateral density difference for an observed temperature difference (δT) is δρ=−αρ0δT(1−r −1). Observations from regions offshore and along the coast of Portugal are used to evaluate the method. Errors of less than 0.05 psu are exhibited in the evaluation of salinity determined from T-5 XBT drops compared with nearly simultaneous CTD casts. A comparison of water properties and cyclostrophic velocities is made using XCP temperatures and XCP velocities in a meddy.

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M. C. Gregg, D. P. Winkel, and T. B. Sanford

Abstract

The Multi-Scale profiler (MSP) resolves shear between vertical wavenumbers of 0.01 cpm and the viscous cutoff of small-scale turbulence. Observations from five sites reveal varied spectral shapes and amplitudes. Spectral amplitudes measured at low latitude do not increase toward the equator, contrary to Munk, and their shapes differ from the Garrett and Munk model by having weak maxima between 0.02 and 0.05 cpm. Moreover, at wavenumbers larger than 0.1 cpm these spectra roll off more steeply, k 3 −1, than do spectra at midlatitude. Of two average spectra from midlatitude, one is close to the Garrett and Munk model at low wavenumbers, and at 0.1 cpm it begins to roll off as k 3 −1. The second midlatitude spectrum has amplitudes well above Garrett and Munk at low wavenumbers, begins a k 3 −1 rolloff near 0.04 cpm, and has a well-developed turbulent range near 1 cpm. The decrease in the start of the rolloff is not linearly proportional to the increase in spectral amplitude at low wavenumber, unlike the spectra observed by Duda and Cox and models proposed by Munk and also Garrett. In spite of the diversity of shapes and amplitudes at low wavenumbers, all shear spectra have nearly the same amplitude at 0.14 cpm, which is in the rolloff range. The rolloff range cannot be a buoyancy subrange of three-dimensional turbulence because the largest overturns occur only at the high-wavenumber end of the range. Rather, the rolloff must be the signature of the high-wavenumber decay of the internal wave field. Near 0.5kE = (N 3/ε)1/2 spectra change from the internal wave rolloff to the turbulent dissipation range, which is adequately represented by Nasmyth's “universal” spectrum. Midlatitude spectra with amplitudes close to the Garrett and Munk model have very weak turbulent spectra, but those with substantially larger low-wavenumber amplitudes have well-developed turbulent spectra with distinct inertial subranges. Owing to their steeper rolloffs, the low-latitude records also have weak dissipation spectra even though their spectra rise above Garrett and Munk at wavenumbers slightly less than the rolloff.

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D. P. Winkel, M. C. Gregg, and T. B. Sanford

Abstract

The Multi-Scale Profiler (MSP), a freely falling dropsonde, has been used over the past 12 years to measure oceanic shear variance. Complete resolution of oceanic shear spectra is achieved by combining the measurements of MSP’s acoustic current meter (ACM), electromagnetic current meter (ECM), and airfoil probes. The ACM detects flow relative to MSP, so the platform motion must be known to determine the water velocity. The vechicl's tilt oscillation is inferred from accelerometer data, and its gross (point mass) horizontal motion is simulated by modeling MSP's response to the relative flow. Forcing on its tail array causes MSP to react as a point mass to fluctuations with scales as small as 2-3 m. The model of Hayes et al. for the TOPS dropsonde was modified so that it reasonably parameterized the large MSP tail force. Relevant dynamics and data processing are discussed, and the point-mass model is presented along with the analytic transfer functions that are used to select parameter values, assess sensitivities, and estimate uncertainties. Because they are unaffected by MSP's horizontal motion, the ECM measurements directly reflect the flow structure and, consequently, provide an onboard reference against which the large-scale corrections to the ACM measurements are validated. Uncorrected ACM data provide a direct check on the airfoil data, which resolve microscale shear variance to within a factor of 2, aside from some noted exceptions in warm, turbulent waters. The motion-corrected ACM profiles are shown to resolve shear variance to within 10%–15% at vertical scales from over 200 m down to 1 m (with minor anomalies at 5-m and 2-3-m scales).

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R-C. Lien, B. Sanford, and W-T. Tsai

Abstract

Measurements of small-scale vorticity, turbulence velocity, and dissipation rates of turbulence kinetic energy ε were taken in a littoral fetch-limited surface wave boundary layer. Drifters deployed on the surface formed convergence streaks with ∼1-m horizontal spacing within a few minutes. In the interior, however, no organized pattern of velocity, vorticity, or turbulence mixing intensity was found at a similar horizontal spatial scale. The turbulent Langmuir number La was 0.6–1.3, much larger than the 0.3 of the typical open ocean, suggesting comparable importance of wind-driven turbulence and Langmuir circulation. Observed ε are explained by the wind-driven shear turbulence. The production rate of turbulence kinetic energy associated with the vortex force is about 10−7 W kg−1, slightly smaller than that generated by the wind-driven turbulence. The rms values of the streakwise component of vorticity σ ζ|| and the vertical component of vorticity σ ζz have a similar magnitude of ∼0.02 s−1. Vertical profiles of ε, σ ζ||, and σ ζz showed a monotonic decrease from the surface. Traditionally, surface convergence streaks are regarded as signatures of Langmuir circulation. Two large-eddy simulations with and without Stokes drift were performed. Both simulations produced surface convergence streaks and vertical profiles of ε, vorticity, and velocity consistent with observations. The observations and model results suggest that the presence of surface convergence streaks does not necessarily imply the existence of Langmuir circulation. In a littoral surface boundary layer where surface waves are young, fetch-limited, and weak, and La = O(1), the turbulence mixing in the surface mixed layer is primarily due to the wind-driven shear turbulence, and convergence streaks exist with or without surface waves.

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