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John M. Toole

Abstract

Finestructure observations gathered in the equatorial Pacific are discussed. The measurements consist of a 48 h CTD time series within the salinity front at the northern edge of the Equatorial Undercurrent at 110°W. The water column at depths shallower than 150 m at this site was dominated by intense inter-leaving. The vertical temperature and salinity gradient spectra from this depth range are strongly peaked at a wavelength of 20 m and the temperature-salinity anomalies are coherent and in phase at this scale. Yet, unlike the finestructure observed in fronts at higher latitudes, the finestructure is not density-compensating on any scale on account of an excess of temperature variance. Rather than interpret this equatorial finestructure as resulting from nearly isopycnal differential advection, it is suggested that the features may be the signature of low-frequency internal waves where lateral displacements are reflected in the salinity profiles and lateral and vertical displacements induce the temperature variability. If so, lateral mixing across the Equatorial Undercurrent may involve an interaction between internal waves and salt fingering as well as double-diffusively driven intrusions.

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John M. Toole

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Data obtained on two cruises to the Antarctic Polar Front are used to investigate the nature of thermohaline intrusions in the front. These data, obtained in the Drake Passage and south of New Zealand, include CTD time series made relative to neutrally buoyant vertical-current meters, temperature data from these floats, and small-scale CTD sections. Analysis of these data is divided into a study of temporal persistence and an examination of the spatial structure of the intrusions. The study of intrusion time scales is hampered by the presence of large spatial gradients in the intrusion field. This study suggests that features are persistent for several days but additional measurements appear necessary to resolve the intrusion decay process. The length scales of intrusions are found to vary within the frontal zone. Near the region of maximum lateral gradients in the front, length scales of 5 to 10 km are observed, while elsewhere scales of only 1 km are found. It is suggested that these small, isolated leaves of fluid are old intrusions that have been sheared and advected from their generation site. Several pieces of circumstantial evidence are presented to support the hypothesis that intrusions are driven by salt fingering, and good agreement between the observations and the results of a dynamical model of intrusions driven by salt fingers is noted. Finally, application of a statistical model for intrusive variance predicts that intrusions may effect constant lateral eddy diffusivities in fronts provided the small-scale vertical diffusivity is constant.

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John M. Toole
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John M. Toole

Abstract

Current-meter data from a two-year mooring within a fracture zone on the western flank of the Mid-Atlantic Ridge in the South Atlantic Ocean are reported. The mooring, deployed in conjunction with the Brazil Basin Tracer Release Experiment, was placed in the general area where enhanced diapycnal mixing had previously been inferred. The current-meter data characterize the velocity, temperature, shear, and temperature gradient variability as a function of frequency. Energetic velocities and shears were observed at the mooring at a variety of frequencies. In addition to semidiurnal flows, a significant amount of shear variance derived from near-inertial motions, as has been seen in a recent numerical modeling study of tidal-frequency internal wave radiation and wave–wave interaction. At times, a fortnightly modulation of the total superinertial shear variance was indicated in the data, but this signal did not dominate the records. Wave ray tracing indicates that the deeper current meters may have been placed in a shadow zone for locally generated internal tides. At shallower levels, it is suggested that dispersion, wave–wave interaction, and wave breaking effectively obscured the sources of the finescale energy. Average diapycnal diffusivity estimates inferred from a Richardson-number-based parameterization and from observations of temperature inversions at 4648 m were of the same order of magnitude as those derived from turbulent dissipation estimates and from the rate of diapycnal tracer dispersion. The mooring data thus add additional support to the idea that energetic finescale motions above rough bathymetry support enhanced turbulent diapycnal mixing in these regions.

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Eric Kunze and John M. Toole

Abstract

Fine- and microstructure profiles collected over Fieberling Seamount at 32°26′N in the eastern North Pacific reveal a variety of intensified baroclinic motions driven by astronomical diurnal tides. The forced response consists of three phenomena coexisting in a layer 200 m thick above the summit plain: (i) an anticyclonic vortex cap of core relative vorticity − 0.5f, (ii) diurnal fluctuations of ±15 cm s−1 amplitude and 200-m vertical wavelength, and (iii) turbulence levels corresponding to an eddy diffusivity κ e ≅ 10 × 10−4 m2 s−1. The vortex cannot be explained by Taylor–Proudman dynamics because of its − 0.3fN 2 negative potential vorticity anomaly. The ±0.3f fortnightly cycle in the vortex’s strength suggests that it is at least partially maintained against dissipative erosion by tidal rectification. The diurnal motions are slightly subinertial, turning clockwise in time and counterclockwise with depth over the summit plain. They also exhibit a fortnightly cycle in their amplitude, pointing to seamount amplification of impinging barotropic tides. Their horizontal structure resembles that of a seamount-trapped topographic wave. However, the counterclockwise turning with depth of the horizontal velocity vector and the 180° phase difference between radial velocity ur and vertical displacement ξ′ = −T′/T z (producing a net positive radial heat flux 〈ur T′〉) are more consistent with a vortex-trapped near-inertial internal wave of upward energy propagation. The strong negative vorticity of the vortex cap allows the diurnal frequency to be effectively superinertial; that is, diurnal fluctuations satisfy a hyperbolic equation within the vortex. A vortex- trapped wave would encounter a vertical critical layer at the top of the cap where its energy would be lost to turbulence. Observed turbulent kinetic energy dissipation rates of ε = 3 × 10−8 W kg−1 are sufficiently high to deplete the wave and vortex in less than 3 days, emphasizing the strongly forced/damped nature of the system. Inferred eddy diffusivities two orders of magnitude larger than those found in the ocean interior suggest that, locally, seamounts are important sites for diapycnal transport. On basin scales, however, there are too few seamounts or ridges penetrating the main pycnocline to support a basin-averaged diffusivity of O(10−4 m2 s−1) above 3000-m depth.

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John M. Toole and Mark D. Borges

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Pacific near-equatorial absolute-velocity data and CTD–O2 observations collected from the R.V. Conrad in the northern fall of 1982 are presented. Quite by chance, the cruise took place during the early stages of the 1982/83 El Niño. The data obtained are generally consistent with the fundamental ideas about El Niño onset, in that abnormal eastward flow near the surface and large thermocline depressions were observed. These changes are quantified by comparing with previously collected data and are discussed in terms of simple equatorially-trapped wave dynamics. The results indicate that first- and second-mode disturbances, probably Kelvin waves, were prominent in the equatorial Pacific in the fall of 1982.

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Katherine E. Silverthorne and John M. Toole

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Seasonal variability of near-inertial horizontal kinetic energy is examined using observations from a series of McLane Moored Profiler moorings located at 39°N, 69°W in the western North Atlantic Ocean in combination with a one-dimensional, depth-integrated kinetic energy model. The time-mean kinetic energy and shear vertical wavenumber spectra of the high-frequency motions at the mooring site are in reasonable agreement with the Garrett–Munk internal wave description. Time series of depth-dependent and depth-integrated near-inertial kinetic energy are calculated from available mooring data after filtering to isolate near-inertial-frequency motions. These data document a pronounced seasonal cycle featuring a wintertime maximum in the depth-integrated near-inertial kinetic energy deriving chiefly from the variability in the upper 500 m of the water column. The seasonal signal in the near-inertial kinetic energy is most prominent for motions with vertical wavelengths greater than 100 m but observable wintertime enhancement is seen down to wavelengths of the order of 10 m. Rotary vertical wavenumber spectra exhibit a dominance of clockwise-with-depth energy, indicative of downward energy propagation and implying a surface energy source. A simple depth-integrated near-inertial kinetic energy model consisting of a wind forcing term and a dissipation term captures the order of magnitude of the observed near-inertial kinetic energy as well as its seasonal cycle.

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John M. Toole and Stanley P. Hayes

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An analysis of finescale horizontal-velocity shear and density data collected along 110°W longitude in the equatorial Pacific is presented. The measurements were made with the free-fall velocity–density profiler, TOPS. Twenty-five deployments are used to investigate the variability in the depth interval 150–900 m. In this interval the shear and strain fields are dominated by finescale structures. The extra-equatorial latitude band 4–10°N exhibits shear and strain spectra as well as Richardson number (Ri) statistics that are consistent with midlatitude internal-wave model predictions. Approaching the equator, an enhancement of shear and strain variance is found along with an accompanying increase in the occurrence of Ri less than ¼. Consistent with previous studies, the present measurement suggest that the 13°C thermostad is turbulently mixed. A high occurrence of Ri less than ¼ is also found below the thermostad and in the latitude range 2–4°S. The implications of these observations are discussed along the representativeness of the present measurements.

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Kurt L. Polzin, John M. Toole, and Raymond W. Schmitt

Abstract

Fine- and microstructure data from a free fall profiler are analysed to test models that relate the turbulent dissipation rate (ε) to characteristics of the internal wave field. The data were obtained from several distinct internal wave environments, yielding considerably more range in stratification and wave properties than has been previously available. Observations from the ocean interior with negligible large-scale flow were examined to address the buoyancy scaling of ε. These data exhibited a factor of 140 range in squared buoyancy frequency (N 2) with depth and uniform internal wave characteristics, consistent with the Garrettt–Munk spectrum. The magnitude of ε and its variation with N(ε ∼ N 2) was best described by the dynamical model of Henyey et al. A second dynamical model, by McComas and Muller, predicted an appropriate buoyancy scaling but overestimated the observed dissipation rates. Two kinematical dissipation parameterizations predicted buoyancy scalings of N 3/2; these are shown to be inconsistent with the observations.

Data from wave fields that depart from the canonical GM description are also examined and interpreted with reference to the dynamical models. The measurements came from a warm core ring dominated by strong near-inertial shears, a region of steep topography exhibiting high-frequency internal wave characteristics, and a midocean regime dominated al large wavelengths by an internal tide. Of the dissipation predictions examined, those of the Henyey et al. model in which εN −2 scales as E 2, where E is the nondimensional spectral shear level, were most consistent with observations. Nevertheless, the predictions for these cases exhibited departures from the observations by more than an order of magnitude. For the present data, these discrepancies appeared most sensitive to the distribution of internal wave frequency, inferred here from the ratio of shear spectral level to that for strain. Application of a frequency-based correction to the Henyey et al. model returned dissipation values consistent with observed estimates to within a factor of 2.

These results indicate that the kinetic energy dissipation rate (and attendant turbulent mixing) is small for the background Garrett and Munk internal wave conditions (0.25εN −2∼0.7 × 10−5 m2 s−1). Dissipation and mixing become large when wave shear spectral levels are elevated, particularly by high-frequency waves. Thus, internal wave reflection/generation at sleep topographic features appear promising candidates for achieving enhanced dissipation and strong diapycnal mixing in the deep ocean that appears required by box models and advection–diffusion balances.

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Martin Claus, Richard J. Greatbatch, Peter Brandt, and John M. Toole

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The equatorial deep jets (EDJs) are a ubiquitous feature of the equatorial oceans; in the Atlantic Ocean, they are the dominant mode of interannual variability of the zonal flow at intermediate depth. On the basis of more than 10 years of moored observations of zonal velocity at 23°W, the vertically propagating EDJs are best described as superimposed oscillations of the 13th to the 23rd baroclinic modes with a dominant oscillation period for all modes of 1650 days. This period is close to the resonance period of the respective gravest equatorial basin mode for the dominant vertical modes 16 and 17. It is argued that since the equatorial basin mode is composed of linear equatorial waves, a linear reduced-gravity model can be employed for each baroclinic mode, driven by spatially homogeneous zonal forcing oscillating with the EDJ period. The fit of the model solutions to observations at 23°W yields a basinwide reconstruction of the EDJs and the associated vertical structure of their forcing. From the resulting vertical profile of mean power input and vertical energy flux on the equator, it follows that the EDJs are locally maintained over a considerable depth range, from 500 to 2500 m, with the maximum power input and vertical energy flux at 1300 m. The strong dissipation closely ties the apparent vertical propagation of energy to the vertical distribution of power input and, together with the EDJs’ prevailing downward phase propagation, requires the phase of the forcing of the EDJs to propagate downward.

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