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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

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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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Eric Kunze and Thomas B. Sanford

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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

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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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H. Thomas Rossby and Thomas B. Sanford

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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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James B. Girton and Thomas B. Sanford

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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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Thomas B. Sanford and Dus̆an S. Zrnić
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Gregory C. Johnson and Thomas B. Sanford

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Data from a CTD station and three expendable current profiler drops at the center of the sill of the Faroe Bank Channel are used to examine the structure of the northwestward outflow of cold, relatively fresh, dense water from the Norwegian Sea into the Atlantic Ocean. A bottom boundary layer is present and exerts a bottom stress estimated at 3.5 Pa using observations in the log-layer. The shear at the interface between the outflow water and the water above is sufficiently strong to overcome the stratification and generate shear instabilities. The large stress at the bottom boundary creates an Ekman layer and thus a secondary cross-channel flow to the southwest there. A flow of similar magnitude but to the northeast is found in the high shear region at the interface. Hence, these data suggest a spiral velocity pattern in the outflow, created by the Ekman flow in the bottom boundary layer and cross-channel flow at the interface. This proposed circulation scheme explains the pinching of the density field observed at the southwest channel wall in CTD sections across the channel.

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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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Robert J. Serafin and Thomas B. Sanford

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No abstract available.

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