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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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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

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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

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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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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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H. Thomas Rossby and Thomas B. Sanford

Abstract

A time series of velocity profiles derived from three methods are used to describe the variations of current in time and in the vertical. Absolute velocity profiles were Obtained by acoustically tracking a falling probe; relative profiles were derived from motional electric fields (EM method) measured by a second free-fall instrument and from density observations using the dynamic method. The two free-fall profile methods agree within 0.01 m s−1 rms averaged over depth intervals in which the observations were separated in time by less than 10 min. Although the rms differences between profiles increases to about 0.02 m s−1, due to the fact that one device falls at one-third the speed of the other, the agreement between methods was sufficiently good that the eight acoustic profiles and six EM profiles were combined to yield a time series lasting 4 days. These profiles, taken near Bermuda In May 1971, were divided into two sets having a mean time separation of 2 days. Each set of profiles was fitted to a time-mean or steady profile and a rotary component of inertial frequency. Using lagged correlation and vector spectral analysis, it is shown that the inertial energy propagates downward at a group velocity having a vertical component of about 0.5 mm s−1. These results suggest a surface or near-surface energy source and a lack of modal structure to the inertial currents. The steady component agrees within 0.02 m s−1 rms with the geostrophic profile computed every 200 m and both have the same shear over the interval 200–1200 m.

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Mark D. Prater and Thomas B. Sanford

Abstract

A meddy, an eddy formed from Mediterranean source water, was surveyed in detail with two types of expendable profilers and a CTD instrument. The muddy comprised two distinct, vertically aligned tenses with a combined thickness of 650 m. Both lenses were stratification minima. The upper lens was warmer, fresher (12.25°C, 36.5 psu), and more circular., the lower lens was cooler, more saline (12.1°C, 36.65 psu), and more elliptical, oriented alone a northeast by southwest line. The upper lens, homogeneous out to a radius of 6 km, had a radius of maximum velocity of 9 km. Its relative vorticity was −0.85 f, and its Ertel potential vorticity, 4 × 10−12 (m s)−1, was 17 times below ambient levels due to the combined effects of negative relative vorticity and vortex stretching. The meddy contained more kinetic energy than available potential energy (energy Burger number of 2.5). Compared with historical meddies, it had a larger Burger number and a more negative vorticity Rossby number.

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Robert J. Serafin and Thomas B. Sanford

Abstract

No abstract available.

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