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Gregory C. Johnson, Rolf G. Lueck, and Thomas B. Sanford

Abstract

Bottom and interfacial stresses on the Mediterranean outflow plume are estimated using vertical profiles of turbulent dissipation and velocity collected in the Gulf of Cadiz. Turbulent dissipation is high throughout the plume, with a local minimum often present near the plume nose (depth of maximum downstream velocity). Bottom stresses are estimated by applying a log-layer. model to the dissipation measurements. The dissipation measurements are also divided by plume-scale vertical shear from the horizontal velocity profiles to construct profiles of stress within the plume. The mean stress estimates in the bottom layer agree well with those calculated in the log layer from the dissipation measurements alone. The bottom-layer means are slightly larger than those of the interfacial layer. The maximum stresses in each layer are uncertain, since they depend on the ill-defined shape of the stress profiles within the plume. Dissipation-derived stress estimates in the log layer and those from dissipation measurements combined with the plume-scale vertical shear of horizontal velocity are roughly one-third the magnitude of those made in the log layer from velocity measurements and those made in the interfacial layer from the residuals of bulk mass and volume budgets (Part I). Possible reasons for this discrepancy are advanced.

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David P. Winkel, Michael C. Gregg, and Thomas B. Sanford

Abstract

Measurements by the Multi-Scale Profiler (MSP) at seven stations spanning the Straits of Florida characterize levels and patterns of internal wave activity and mixing in this vertically sheared environment. Contrasting properties suggest five mixing regimes. The largest and most vast is the interior regime, where the background flow has an inverse Richardson number (Ri−1) ranging up to 0.55, shear is dominated by fluctuations that are 1–4 times stronger than in the open ocean, and turbulent diffusivities are similarly moderate at (1–4) × 10−5 m2 s−1. The high-velocity core of the current, near the surface at midchannel, is associated with weak mixing. To its west is a zone of high mean shear, where strong stratification results in background Ri−1 of only 0.4, fluctuations are weak, and diffusivity is moderate. Intermittent shear features beneath the core have mean Ri−1 > 1 and strong turbulence. Two regimes are related to channel topography. Adjacent to the steep eastern slope, finescale shear is predominately cross-channel, and turbulence varies from nearly the weakest to nearly the strongest. Within 100 m of the channel floor, turbulent stratified boundary layers are mixing at (2–6) × 10−4 m2 s−1 to account for one-half of the section-averaged diffusivity. Using existing finescale parameterizations, observed dissipation rates can be predicted within a factor of 2 for most of this dataset, despite significantly strong mean shear and generally anisotropic and asymmetric fluctuations. The exceptions are in the high mean shear zones, where total rather than fluctuating shear yields reasonable estimates, and in some of the more turbulent regions, where shear underestimates mixing. Given its overall moderate levels of turbulence and finescale shear, the Florida Current is not a hot spot for oceanic mixing.

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Thomas B. Sanford, Robert G. Driver, and John H. Dunlap

Abstract

A freely failing current meter called the Absolute Velocity Profiler (AVP) is described. This profiler is an expansion of a previously developed instrument, the Electro-Magnetic Velocity Profiler (EMVP), with the additional capability of acoustic Doppler (AD) measurements to determine the reference velocity for the EM profiles. The AVP measures the motional electric currents in the sea and the Doppler frequency shin of bottom-scattered echoes. The EM measurements yield a profile of the horizontal components of velocity relative to a depth-independent reference velocity; the AD measurements determine the absolute velocity of the AVP with respect to the seafloor. The EM profile is obtained from the sea surface to the bottom, and the AD measurements are obtained within about 60–300 m of the seafloor. The combination of the EM and AD measurements yields an absolute velocity profile throughout the water column. Performance analyses show the method is accurate to within 1–2 cm s−1 rms. The profiler also measures temperature and its gradient.

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Eric Kunze, Maureen A. Kennelly, and Thomas B. Sanford

Abstract

High-resolution velocity profiles to depths of 1600 m were collected off Point Arena and near Pioneer Seamount, California. They reveal shear increasing with depth relative to the GM76 model spectrum. Using an empirical parameterization found to collapse microstructure data the “dissipation rate” and “eddy diffusivity” are estimated from these shears. Away from the seamount, dissipation rates are depth invariant at 3–6(×10−10 W kg−1). As a result, the eddy diffusivity increases with depth, approaching 0.2–0.3(×10−4 m2 s−1) below 1200-m depth. This may be a result of the proximity of the continental rise and sloping topography, but there is evidence that it is a general result for the abyssal ocean. Immediately above the seamount, there is a 300–400-m thick layer of elevated shear, corresponding to an eddy diffusivity of ∼ 10−4 m2 s−1. If this localized enhancement is typical of seamounts, topographically induced mixing is insufficient to significantly modify global average mixing.

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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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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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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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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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Gregory C. Johnson, Thomas B. Sanford, and Molly O’Neil Baringer

Abstract

In September 1988 six sections were occupied across the Mediterranean outflow plume in the Gulf of Cadiz within 100 km of the Strait of Gibraltar. Vertical profiles of temperature and salinity were collected at CTD stations. Velocity and temperature profiles were collected with expendable current profilers at a subset of these stations. At the channel base, the plume undergoes geostrophic adjustment and turns northwest to flow along the continental slope. There it decelerates and spreads gradually down the slope as friction slows the current and allows it to cross isobaths. Within the plume, downstream velocity and density increase rapidly in the interfacial layer with depth to the velocity maximum, or nose, 5–150 m above the bottom. Below the nose, in the bottom layer, downstream velocity decreases rapidly toward the bottom, but the stratification is weak. Ekman-like veering occurs in the interfacial layer. Local bottom stresses on the plume are estimated by fitting the near-bottom velocity profiles to a log-layer model. These stresses are compared with bulk estimates of total stresses from momentum budget residuals and of interfacial stresses from combining the mean vertical shear with bulk turbulent dissipation estimates. The downstream pattern of the sum of the local bottom stresses and the bulk interfacial stresses agrees well in magnitude and distribution with that of the bulk total stresses. The largest stresses reach a mean of 5 Pa where the plume is flowing rapidly westward down a channel after exiting the strait, thinning, and accelerating. These stresses are an order of magnitude larger than mean wind stress values over the ocean gyres and exceed most bottom stress estimates in other regions.

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James F. Price, Thomas B. Sanford, and George Z. Forristall

Abstract

The upper ocean's response to three hurricanes [Norbert (1984), Josephine (1984) and Gloria (1985)] is examined using field observations and a numerical ocean model. Our goal is to describe the physical processes that determine the structure and amplitude of hurricane-driven upper-ocean currents.

All three of these Northern Hemisphere hurricanes produced a rightward-biased response of the mixed-layer current and transport. This asymmetry arises because the wind stress vector rotates clockwise on the right side of the track and remains nearly parallel with the inertially rotating mixed-layer current during most of the hurricane passage. The maximum observed mixed-layer current varied from 0.8 m s−1 in response to Josephine, which was a large but comparatively weak hurricane, to 1.7 m s−1 in response to Gloria, which was very large and also intense.

These cases have been simulated with a three-dimensional numerical model that includes a treatment of wind-driven vertical mixing within the primitive equations. The simulations give a fairly good representation of the horizontal pattern and amplitude of the mixed-layer current, accounting for over 80% of the variance of the observed current. Model skill varies considerably with the amplitude of the mixed-layer current, being much higher for stronger currents than it is for weaker currents. This and other evidence suggest that a major contributor to the difference between the observed and simulated currents may be a noise component of the observed current that arises from measurement and analysis error and from prehurricane currents.

The Norbert case was distinguished by a large Burger number, ∼1/2, which is a measure of pressure coupling between the forced stage mixed-layer currents and the relaxation stage thermocline currents. The observations and the simulation show upwelling of up to 25 m and strong thermocline-depth currents up to 0.3 m s−1 under the rear half of Norbert. Thermocline currents have a very simple vertical structure, a monotonic decay with increasing depth, and nearly constant direction. Their horizontal structure is more complex but appears to be due to an acceleration toward a low pressure anomaly associated with the first upwelling peak about 100 km behind the eye of Norbert.

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