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Eric Kunze and Thomas B. Sanford

Abstract

Velocity measurements near Caryn Seamount in the Sargasso Sea reveal intensified mean and near-inertial motions in the upper ocean. Embedded in a regional eastward flow is an anticyclonic eddy that appears to be tied to the topography. An energetic downward-propagating near-inertial wave packet is found at the eddy's base. This wave appears to be trapped in the eddy, undergoing critical-layer amplification as it tries to leave. Near the bottom, enhanced upgoing near-inertial wave energy is found within 1000 m of the top of the seamount.

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Eric Kunze and Thomas B. Sanford

Abstract

The prevailing view that submesoscale fluctuations (horizontal wavelengths less than a few kilometers and vertical wavelengths less than a few hundred meters) are dominated by internal gravity waves is tested by measuring Ertel's potential vorticity, Π = (f + ∇ × V) · ∇B, where the buoyancy B = −gδρ/ρo. Unlike geostrophic or nonlinear Ertel vorticity-carrying motions, internal waves have no Ertel vorticity fluctuations. Velocity and temperature profile surveys beside Ampere Seamount reveal appreciable Ertel enstrophy, and thus a significant non-internal-wave component, on horizontal wavelengths of 6–15 km and vertical wavelengths of 50–380 m. The twisting terms are negligible and the relative vorticities less than 0.2f, so the anomalies are in geostrophic balance.

It is unlikely that the anomalies arise from stirring of the large-scale isopycnal gradients of stretching and planetary Ertel vorticity as this would require stirring lengths of thousands of kilometers. The most likely source appears to be forcing at the seamount, but generation by (i) dissipative 3D turbulence in the pycnocline or (ii) detrainment of the winter mixed layer cannot be absolutely ruled out. It remains to determine whether the coexistence of internal wave and Ertel vorticity-carrying fluctuations characterizes smaller scales (λ ≤ 50 m, λH ≤ 5 km) in the deep ocean away from topography as well.

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Eric Kunze and Thomas B. Sanford

Abstract

A parameterization based on internal wave/wave interaction theory, which infers turbulence production from finescale internal wave shear, is applied to 114 full-water-depth velocity profiles in the Sargasso Sea. An average eddy diffusivity of 0.1 × 10−4 m2 s−1, independent of depth, is inferred. This value is consistent with full-water-depth microstructure measurements from abyssal basins in the eastern North Atlantic and eastern North Pacific. It is an order of magnitude smaller than the values inferred from a simple vertical advection-diffusion balance or bulk budgets. Thus, the mixing needed to close deep global water-mass budgets does not appear to occur over midlatitude abyssal plains. This suggests that ocean mixing is either (i) confined to boundary layers as in ideal thermocline theory or (ii) localized to hotspots, such as over rough topography or restrictive passages. Abyssal diffusivities do not display any dependence on bottom slope for slopes less than 7 × 10−2 based on 5–10 km bathymetry, but are higher over convex than concave topography and higher in stronger bottom currents.

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Eric Kunze and Thomas B. Sanford

Abstract

Near-inertial with horizontal scales ∼O(10 km) dominate profiles of velocity finestructure collected in the North Pacific Subtropical Front during January 1980. Considerable spatial variability is observed. Two features in particular contain most of the energy: a 20 cm s−1 amplitude (λz = 100 m) wave on the warm edge of the front propagating downward and away from the front, and a low wavenumber (λz = 500 m) wave reflecting off the surface. The propagating wavegroup is four times as energetic as the local downgoing near-inertial wave field. Its spatial structure is not consistent with propagation in a homogeneous medium, which suggests that it may be interacting with the front. Possible mechanisms for the existence and properties of the wavegroup are discussed, including baroclinic/barotropic instability, wind-forcing and enhancement by wave-mean flow interaction. a wave-mean flow interaction model that predicts trapping and amplification of near-inertial flow interaction. A wave-mean flow interaction model that predicts trapping and amplification of near-inertial waves in regions of negative vorticity reproduces the observed features most consistently.

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James B. Girton and Thomas B. Sanford

Abstract

Bulk properties of the Denmark Strait overflow (DSO) plume observed in velocity and hydrography surveys undertaken in 1997 and 1998 are described. Despite the presence of considerable short-term variability, it is found that the pathway and evolution of the plume density anomaly are remarkably steady. Bottom stress measurements show that the pathway of the plume core matches well with a rate of descent controlled by friction. The estimated entrainment rate diagnosed from the rate of plume dilution with distance shows a marked increase in entrainment at approximately 125 km from the sill, leading to a net dilution consistent with previous reports of a doubling of overflow transport measured by current meter arrays. The entrainment rate increase is likely related to the increased topographic slopes in the region, compounded by a decrease in interface stratification as the plume is diluted and enters a denser background.

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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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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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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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Charles L. Johnson and Thomas B. Sanford

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0bservations of vertical profiles of horizontal velocity made around the island of Bermuda during the FAME experiment reveal anomalies in the internal wave field associated with the island bathymetry. Compared to similar data taken at open-ocean sites, the near-Bermuda data exhibit lower horizontal kinetic energy levels, especially in the near inertial frequency band. Also, whereas the open ocean data consistently show the dominance of clockwise over anticlockwise polarized (with depth) energy, implying a near surface energy source, the Bermuda profiles frequently consist of mostly anticlockwise polarized energy. The near island internal wave field possesses a significant inshore over alongshore shear anisotropy, which, together with the anticlockwise polarization, might signify energy generation at the sea bottom with subsequent propagation upward radially away from the island. No relation is found between the amount of shear anisotropy and the energy level of the wave field. The shear anisotropy and temperature finestructure appear to be related to horizontal shear of the time-mean current and proximity to the island.

During the period when a large-scale eddy was impinging on the island, significant coherence was observed between vertical gradients of temperature and velocity components in the wavelength band 30–50 m. The phase between temperature and velocity variations was consistent with nearshore internal wave generation followed by upward and outward propagation. The observed coherence and phase are compared to that expected from a horizontally anisotropic, vertically asymmetric internal wave field. It is found that such a wave field can account for the observed temperature finestructure by internal wave distortion of the time-mean vertical temperature gradient. In contrast, horizontal advection by internal waves of a time-mean horizontal temperature gradient produces insignificant temperature finestructure. The narrow band coherence presumably results from scale-dependent generation or propagation.

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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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