Low-mode Internal Tides Research Articles

The Latitudinal Dependence of Shear and Mixing in the Pacific Transiting the Critical Latitude for PSI

AbstractTurbulent mixing rates are inferred from measurements spanning 25°–37°N in the Pacific Ocean. The observations were made as part of the Internal Waves Across the Pacific experiment, designed to investigate the long-range fate of the low-mode internal tide propagating north from Hawaii. Previous and companion results argue that, near a critical latitude of 29°N, the internal tide loses energy to high-mode near-inertial motions through parametric subharmonic instability. Here, the authors estimate mixing from several variations of the finescale shear–strain parameterization, as well as Thorpe-scale analysis of overturns. Though all estimated diffusivities are modest in magnitude, average diffusivity in the top kilometer shows a factor of 2–4 elevation near and equatorward of 29°N. However, given intrinsic uncertainty and the strong temporal variability of diffusivity observed in long mooring records, the meridional mixing pattern is found to be near the edge of statistical significance.

Mapping Low-Mode Internal Tides from Multisatellite Altimetry

Mapping Low-Mode Internal Tides from Multisatellite Altimetry

Incoherent Nature of M2 Internal Tides at the Hawaiian Ridge

Abstract Moored current, temperature, and conductivity measurements are used to study the temporal variability of M2 internal tide generation above the Kaena Ridge, between the Hawaiian islands of Oahu and Kauai. The energy conversion from the barotropic to baroclinic tide measured near the ridge crest varies by a factor of 2 over the 6-month mooring deployment (0.5–1.1 W m−2). The energy flux measured just off the ridge undergoes a similar modulation as the ridge conversion. The energy conversion varies largely because of changes in the phase of the perturbation pressure, suggesting variable work done on remotely generated internal tides. During the mooring deployment, low-frequency current and stratification fluctuations occur on and off the ridge. Model simulations suggest that these variations are due to two mesoscale eddies that passed through the region. The impact of these eddies on low-mode internal tide propagation over the ridge crest is considered. It appears that eddy-related changes in stratification and perhaps cross-ridge current speed contribute to the observed phase variations in perturbation pressure and hence the variable conversion over the ridge.

A laboratory study of low-mode internal tide scattering by finite-amplitude topography

We present the first laboratory experimental results concerning the scattering of a low-mode internal tide by finite-amplitude Gaussian topography. Experiments performed at the Coriolis Platform in Grenoble used a recently conceived internal wave generator as a means of producing a high-quality mode-1 wave field. The evolution of the wave field in the absence and presence of a Gaussian was studied by performing spatiotemporal modal decompositions of velocity field data obtained using particle image velocimetry. The results support the belief that finite-amplitude topography produces significant reflection of the internal tide and transfer of energy from low to high modes.

Low-mode internal tide generation by topography: an experimental and numerical investigation

We analyse the low-mode structure of internal tides generated in laboratory experiments and numerical simulations by a two-dimensional ridge in a channel of finite depth. The height of the ridge is approximately half of the channel depth and the regimes considered span sub- to supercritical topography. For small tidal excursions, of the order of 1% of the topographic width, our results agree well with linear theory. For larger tidal excursions, up to 15% of the topographic width, we find that the scaled mode 1 conversion rate decreases by less than 15%, in spite of nonlinear phenomena that break down the familiar wave-beam structure and generate harmonics and inter-harmonics. Modes two and three, however, are more strongly affected. For this topographic configuration, most of the linear baroclinic energy flux is associated with the mode 1 tide, so our experiments reveal that nonlinear behaviour does not significantly affect the barotropic to baroclinic energy conversion in this regime, which is relevant to large-scale ocean ridges. This may not be the case, however, for smaller scale ridges that generate a response dominated by higher modes.

Continuously stratified nonlinear low-mode internal tides

Author Posting. © Sears Foundation for Marine Research, 2008. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 66 (2008): 299-323, doi:10.1357/002224008786176025.

Spectral modification and geographic redistribution of the semi-diurnal internal tide

A realistic-geometry global baroclinic tidal model forced with a single tidal constituent ( M 2 ) is used to investigate the generation of the internal tide and the associated radiated baroclinic energy flux. The model internal wave spectrum is populated at discrete frequency multiples ( 1 / 2 , 1 , 3 / 2 , 2 , 5 / 2 , … ) of M 2 . The 1 / 2 M 2 subharmonic is particularly energetic at its turning latitude of ± 28.8 ° . Poleward only integer superharmonics of M 2 are significantly excited. The subharmonic turning latitude (SHTL) disturbance has high vertical wavenumber and shear, provided that internal tide energy level exceeds a threshold value. Under these circumstances, Richardson numbers smaller than 1/4 occur in the upper few hundred meters in both the realistic-geometry model and in a complimentary idealized geometry two-dimensional (2D) model. In the 2D model, the disturbance enables Richardson number dependent diapycnal entrainment to effect a modification of the stratification of the upper 400 m of the ocean, and poleward cross-SHTL energy flux falls to 10% of its pre-instability value due to energy transfer to the non-propagating (i.e., inertial) subharmonic. Realistic-geometry simulations suggest a more modest 40% decrease in net flux, although the strongest beams are almost entirely shut down. The predicted energy flux-convergence implies a thermocline dissipation rate in the 28.5–30.0°N latitude band of 5 × 10 - 9 W kg - 1 , with an associated diapycnal diffusivity of 10 - 4 m 2 s - 1 . North of Hawaii the implied regional dissipation rate reaches 4 × 10 - 8 W kg - 1 with an associated thermocline diffusivity of 8 × 10 - 4 m 2 s - 1 . Investigations of subgridscale parameterization and resolution sensitivity suggest that the basic character and magnitude of the predictions are robust to details of the numerical solutions. The present results are taken as further evidence that an increase in shear-driven turbulent mixing in the upper ocean is predicted at special latitudes. It is suggested that the search should be directed to regions where intense low-mode internal tide beams cross their subharmonic turning latitude.

Global Patterns of Low-Mode Internal-Wave Propagation. Part II: Group Velocity

Abstract Using a set of 80 globally distributed time series of near-inertial and semidiurnal energy E and energy flux F computed from historical moorings, the group velocity ĉg ≡ FE−1 is estimated. For a single free wave, observed group speed |ĉg| should equal that expected from linear wave theory. For comparison, the latitude dependence of perceived group speed for perfectly standing waves is also derived. The latitudinal dependence of observed semidiurnal |ĉg| closely follows that expected for free waves at all latitudes, implying that 1) low-mode internal tides obey linear theory and 2) standing internal-tidal waves are rare in the deep ocean for latitudes equatorward of about 35°. At higher latitudes, standing waves cannot be easily distinguished from free waves using this method because their expected group speeds are similar. Near-inertial waves exhibit scattered |ĉg| values consistent with the passage of events generated at various latitudes, with implied frequencies ω ≈ 1.05–1.25 × f, as typically observed in frequency spectra.

Mapping and Wavenumber Resolution of Line-Integral Data for Observations of Low-Mode Internal Tides

Abstract Techniques are developed for using line-integral tomography data to estimate the spectra, maps, and energy of low-mode internal-tide radiation; the extension of these techniques to other phenomena is obvious. Sparse arrays of line integrals over paths 300–1000 km long can generally determine the direction of propagation of semidiurnal radiation well, but the magnitude of the wavenumbers is ambiguous because of sidelobes in the spectrum. Both wavenumber magnitude and direction can generally be determined for diurnal internal-tide radiation. Spectra for the semidiurnal and diurnal internal tides are estimated for the region of the Atlantic Ocean between Puerto Rico and Bermuda using data obtained during the Acoustic Mid-Ocean Dynamics Experiment (AMODE) in 1991–92. Simulations of semidiurnal internal-tide radiation, consisting of wave packets or highly irregular wave crests, are used to show that the line-integral data provide better mapping resolution than point data, but the best results are, of course, obtained when both types of data are used. As a practical example of the formalism of these simulations, maps of M2 internal-tide variability are derived from the AMODE tomography data. Because the inverse problem is underdetermined with the sparse arrays that are deployed in the ocean, the inverse solution generally underestimates the energy of the radiative field. In the simulations employed here, the energy is underestimated by 33% ± 10%, but the exact amount by which the energy is underestimated is dependent on the assumptions made for the simulations, such as the array geometry and the nature of the tidal variability.

New Fellows of the Acoustical Society of America

Fall 2006 Meeting, joint with the Acoustical Society of Japan (Honolulu, Hawaii). The Technical Committee on Acoustical Oceanography AO sponsored two special sessions: 1 “Acoustic scattering by aquatic organisms,” organized by Ken Foote Woods Hole Oceanographic Institution and Masahiro Furusawa from the Acoustical Society of Japan; and 2 “Ocean acoustic tomography for coastal and deep water applications,” organized by Jim Mercer Applied Physical Laboratory, Seattle and Arato Kaneko from the Acoustical Society of Japan. I want to thank Jim and Ken for their extra efforts in coordinating these sessions with their co-chairs from Japan. The Best Student Paper Awards in Acoustical Oceanography went to Paul Roberts of Scripps first prize winner for his paper, “Multiple-angle acoustic scattering and classification of zooplankton,” and Tadanori Fujino Fisheries Research Agency, Hasaki, Japan who won second prize for his paper, “Target strength of a mesopelagic fish Maurolicus japonicus at 38 and 120 kHz.” Andone Lavery Woods Hole Oceanographic Institution represented AO at the Technical Program Organizing Meeting. Spring 2007 Meeting (Salt Lake City, Utah). The 2007 Medwin Prize in Acoustical Oceanography was awarded to Brian Dushaw of the University of Washington, Applied Physical Laboratory, Seattle, for his research on acoustic tomography to measure temperature change in the ocean. Dr. Dushaw presented the AO Prize Lecture entitled, “The recent history of our understanding of low-mode internal tides in the ocean.” AO sponsored two special sessions and a memorial session for Hank Medwin. Jim Lynch Woods Hole Oceanographic Institution organized a special session entitled “Acoustic sensing of the ocean and seabed using gliders and AUVs.” Grant

A Baroclinic Tide in the Eastern North Pacific Determined from 1000-km Acoustic Transmissions*

Abstract In the summer of 1989 a 1000-km acoustic experiment in the eastern North Pacific showed a semidiurnal variation in pulse travel time that was coherent in depth and varied systematically across time fronts. The effect of a single-plane wave, gravest mode, M2, internal tide has been examined by ray tracing and has been found to show the same characteristic variation as the data if the internal tide is coming from the direction of the Gulf of Alaska. The most reasonable fit corresponds to an energy flux of 800 W m−1; implications for tidal dissipation are discussed. In the process of fitting specific acoustic data, the authors have developed an analysis technique for low-mode internal tides that can be applied generally to any acoustic tomography experiment.

Evidence for coastal generation of internal tides from deep-water acoustic observations

In the summer of 1989 a 1000-km acoustic experiment in the Pacific showed a semidiurnal variation in pulse travel time that was coherent in depth and varied systematically across timefronts. The effect of a single-plane-wave, gravest mode, M2, internal tide has been examined by ray tracing and has been found to show the same characteristic variation if the internal tide is coming from the direction of the Gulf of Alaska. In the process of fitting this specific acoustic data, an analysis technique for low-mode internal tides has been developed that can be applied generally to any acoustic tomography experiment.