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  • 1
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    Internal waves in the Arctic : influence of ice concentration, ice roughness, and surface layer stratification (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 123 (2018): 5571-5586, doi:10.1029/2018JC014096.
    Description: The Arctic ice cover influences the generation, propagation, and dissipation of internal waves, which in turn may affect vertical mixing in the ocean interior. The Arctic internal wavefield and its relationship to the ice cover is investigated using observations from Ice‐Tethered Profilers with Velocity and Seaglider sampling during the 2014 Marginal Ice Zone experiment in the Canada Basin. Ice roughness, ice concentration, and wind forcing all influenced the daily to seasonal changes in the internal wavefield. Three different ice concentration thresholds appeared to determine the evolution of internal wave spectral energy levels: (1) the initial decrease from 100% ice concentration after which dissipation during the surface reflection was inferred to increase, (2) the transition to 70–80% ice concentration when the local generation of internal waves increased, and (3) the transition to open water that was associated with larger‐amplitude internal waves. Ice roughness influenced internal wave properties for ice concentrations greater than approximately 70–80%: smoother ice was associated with reduced local internal wave generation. Richardson numbers were rarely supercritical, consistent with weak vertical mixing under all ice concentrations. On decadal timescales, smoother ice may counteract the effects of lower ice concentration on the internal wavefield complicating future predictions of internal wave activity and vertical mixing.
    Description: Seagliders Grant Number: N00014‐12‐10180; Deployment and subsequent analysis efforts of the ITP‐Vs Grant Numbers: N00014‐12‐10799, N00014‐12‐10140; Joint Ocean Ice Studies cruise; Beaufort Gyre Observing System
    Description: 2019-02-14
    Keywords: Internal waves ; Arctic ; Near‐inertial ; Ice roughness ; Ice concentration
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
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    Internal tidal modal ray refraction and energy ducting in baroclinic Gulf Stream currents (2018)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 1969-1993, doi:10.1175/JPO-D-18-0031.1.
    Description: Upstream mean semidiurnal internal tidal energy flux has been found in the Gulf Stream in hydrodynamical model simulations of the Atlantic Ocean. A major source of the energy in the simulations is the south edge of Georges Bank, where strong and resonant Gulf of Maine tidal currents are found. An explanation of the flux pattern within the Gulf Stream is that internal wave modal rays can be strongly redirected by baroclinic currents and even trapped (ducted) by current jets that feature strong velocities above the thermocline that are directed counter to the modal wavenumber vector (i.e., when the waves travel upstream). This ducting behavior is analyzed and explained here with ray-based wave propagation studies for internal wave modes with anisotropic wavenumbers, as occur in mesoscale background flow fields. Two primary analysis tools are introduced and then used to analyze the strong refraction and ducting: the generalized Jones equation governing modal properties and ray equations that are suitable for studying waves with anisotropic wavenumbers.
    Description: The Woods Hole research was supported by National Science Foundation Grant OCE-1060430 and by the Office of Naval Research Grants N00014-11-1-0701 and N00014-17-1-2624. The USM research was supported by ONR Grant N00014-15-1-2288 and National Science Foundation Grant OCE-1537449.
    Description: 2019-02-28
    Keywords: Internal waves ; Wave properties ; Tides ; Differential equations ; Numerical analysis/modeling
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 3
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    Optimal transient growth in thin-interface internal solitary waves (2018)
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 840 (2018): 342-378, doi:10.1017/jfm.2018.19.
    Description: The dynamics of perturbations to large-amplitude Internal Solitary Waves (ISW) in two-layered ows with thin interfaces is analyzed by means of linear optimal transient growth methods. Optimal perturbations are computed through direct-adjoint iterations of the Navier-Stokes equations linearized around inviscid, steady ISWs obtained from the Dubreil-Jacotin-Long (DJL) equation. Optimal perturbations are found as a function of the ISW phase velocity c (alternatively amplitude) for one representative stratification. These disturbances are found to be localized wave-like packets that originate just upstream of the ISW self-induced zone (for large enough c) of potentially unstable Richardson number, Ri 〈 0:25. They propagate through the base wave as coherent packets whose total energy gain increases rapidly with c. The optimal disturbances are also shown to be relevant to DJL solitary waves that have been modi ed by viscosity representative of laboratory experiments. The optimal disturbances are compared to the local WKB approximation for spatially growing Kelvin-Helmholtz (K-H) waves through the Ri 〈 0:25 zone. The WKB approach is able to capture properties (e.g., carrier frequency, wavenumber and energy gain) of the optimal disturbances except for an initial phase of non-normal growth due to the Orr mechanism. The non-normal growth can be a substantial portion of the total gain, especially for ISWs that are weakly unstable to K-H waves. The linear evolution of Gaussian packets of linear free waves with the same carrier frequency as the optimal disturbances is shown to result in less energy gain than found for either the optimal perturbations or the WKB approximation due to nonnormal effects that cause absorption of disturbance energy into the leading face of the wave. Two-dimensional numerical calculations of the nonlinear evolution of optimal disturbance packets leads to the generation of large-amplitude K-H billows that can emerge on the leading face of the wave and that break down into turbulence in the lee of the wave. The nonlinear calculations are used to derive a slowly varying model of ISW decay due to repeated encounters with optimal or free wave packets. Field observations of unstable ISW by Moum et al. (2003) are consistent with excitation by optimal disturbances.
    Description: PYP and BLW acknowledge the support by the National Science Foundation Grant Number OCE-1155558 and OCE{1736989. KRH acknowledges support from Independent Research and Development and Investment in Science Program awards from the Woods Hole Oceanographic Institution.
    Keywords: Solitary waves ; Internal waves ; Instability
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 4
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    Internal waves, finestructure, mocrostructure, and mixing in the ocean (2018)
    Woods Hole Oceanographic Institution
    Details
    Publication Date: 2022-05-26
    Description: Also published as: Reviews of geophysics and space physics (1979) 17: 1524-1548
    Description: Progress in measuring, interpreting, and understanding oceanic internal gravity waves and fine and microstructure is reviewed; we emphasize the quadrennium 1975-1978. The context is how these subjects contribute to oceanic mixing. The overlap between the areas is examined, as is the relevance of the subjects to other aspects of Present trends and suggestions for future work are included, and we offer some speculation on possible progress during the next quadrennium, which may be substantial especially for finestructure understanding.
    Description: Prepared for the Office of Naval Research under Contracts N00014-76-C-0197; NR 083-004 and N00014-75-C-0502 (to the University of Washington) and for the National Science Foundation under Grant OCE 77-25803.
    Keywords: Internal waves ; Fine-structure constant ; Microstructure
    Repository Name: Woods Hole Open Access Server
    Type: Technical Report
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  • 5
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    Critical layers and the Garrett-Munk spectrum (2018)
    Woods Hole Oceanographic Institution
    Details
    Publication Date: 2022-05-26
    Description: Also published as: Journal of Marine Research 38 (1980): 135~145
    Description: The effects of critical level absorption of oceanic internal waves by a mean flow are estimated using the Garrett and Munk (1975) model spectrum. The horizontal currents of the wave field are found to be more intense perpendicular to the mean flow than parallel to it. The cause of this anisotropy is preferential absorption of waves travelling with the mean flow. However, the current anisotropy is only half as large as would be necessary to explain Frankignoul's (1974) observations. The wave momentum flux lost to critical level absorption is found to be nearly proportional to the mean velocity. When the momentum flux is deposited throughout a 400 m thick shear zone, typical of the main thermocline in the North-west Atlantic, the observed stress-shear relationship would correspond to a wave-induced eddy viscosity of -200 cm2 s-1. The effect of the absorbed momentum on the mean flow is to cause a slow (5 m/day) downward phase propagation and slow broadening of the shear profile.
    Description: Prepared for the Office of Naval Research under Contract N00014-76-C-0197; NR 083~400.
    Keywords: Internal waves
    Repository Name: Woods Hole Open Access Server
    Type: Technical Report
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  • 6
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    Bispectra of internal waves (2018)
    Woods Hole Oceanographic Institution
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    Publication Date: 2022-05-26
    Description: Derived from the equations of motion, the bispectrum of power indicates the rate of energy transfer among components of the internal wave field . This, or any other bispectrum, can be evaluated from weak resonant interaction theory given the wave spectrum. Using the Garrett and Munk model of the deep open ocean internal wave spectrum, the bispectrum of power and the closely related auto-bispectrum of vertical displacements are evaluated numerically with the intention of providing an observational test of the weak interaction theory and its predictions. The resulting levels of the bispectra for typical deep ocean internal waves are generally too low to be observed with any statistical confidence in an experiment of reasonable length and cost.
    Description: Prepared for the National Science Foundation under Grant OCE76-23532 and in part by Grants OCE77-25803 and OCE76-14739.
    Keywords: Internal waves ; Energy transfer
    Repository Name: Woods Hole Open Access Server
    Type: Technical Report
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  • 7
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    On the internal wave variability during the Internal Wave Experiment (IWEX) (2018)
    Woods Hole Oceanographic Institution
    Details
    Publication Date: 2022-05-26
    Description: Also published as: Journal of Geophysical Research 84 (1979): 769-776.
    Description: The relation between internal wave variability and larger and smaller scales of motion is investigated, using the IWEX data set. To investigate the role of internal waves in the vertical diffusion of large scale momentum, the time variability of the vertical flux of horizontal internal wave momentum (estimated from temperature and current data) is compared to that of the mean vertical shear. It is found that internal waves cannot cause a vertical viscosity as large as proposed by Müller (1976), but that the data are too noisy to detect a possible wave‐induced viscosity in absolute value of the order of 10−2 m2 s−1 or less. Similarities in the time behavior of the total internal wave energy and that of the square mean vertical shear suggest that some kind of dynamical coupling exists between internal waves and larger scale flows. There is some evidence that the level of temperature finestructure activity also varies in a related way. An analysis of CTD station data taken during Mode demonstrates the mappability of the finestructure activity, and again suggests a relation with the geostrophic eddy flow.
    Description: Prepared for the Office of Naval Research under Contracts N00014-74-C-0262; NR 083- 004 , N00014-76-C-0197: NR 083-400 and for the National Science Foundation under Grant OCE 74-19782 .
    Keywords: Internal waves
    Repository Name: Woods Hole Open Access Server
    Type: Technical Report
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  • 8
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    A four-dimensional primitive equation model for coupled coastal - deep ocean studies (2018)
    Woods Hole Oceanographic Institution
    Details
    Publication Date: 2022-05-26
    Description: A prototype four-dimensional (x,y,z,t) continental shelf/deep ocean model is described. In its present form, the model incorporates the effects of finite-amplitude topography, advective nonlinearities, and variable stratification and rotation. The model can be forced either directly by imposed atmospheric windstress and surface pressure distributions, and energetic mean currents imposed by the exterior oceanic circulation; or indirectly by initial distributions of shoreward propagating mesoscale waves and eddies. To avoid concerns over the appropriate specification of "open" boundary conditions on the cross-shelf and seaward model boundaries, a periodic channel geometry (oriented along-coast) is used. The model employs a traditional finite-difference expansion in the cross-shelf direction, and a Fourier (periodic) representation in the long-shelf coordinate. A modified sigma coordinate system, and a Chebyshev-tau approximation scheme, are used to incorporate the vertical dependence. The model has been validated against a variety of propagating topographic wave problems. Representative run times and error estimates are given.
    Description: Prepared for the National Aeronautics and Space Administration under Grant NAG 5-8.
    Keywords: Ocean waves ; Internal waves
    Repository Name: Woods Hole Open Access Server
    Type: Technical Report
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  • 9
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    Vertical coherence of the internal wave field from towed sensors (2018)
    Woods Hole Oceanographic Institution
    Details
    Publication Date: 2022-05-26
    Description: Also published as: Journal of Physical Oceanography 9 (1979): 518-530
    Description: Constant depth and isopycnal‐following tows are used to estimate the, towed vertical coherence of the internal wave field, at vertical separations of 8.5, 18, 28 and 70 m. The depths of the tows are ∼750 m at the maximum of the buoyancy frequency in the main thermocline of the Sargasso Sea, and near 350 m in the buoyancy frequency minimum between the main and seasonal thermoclines. The towed spectra and towed vertical coherence are compared with three model spectra (GM75, GM76 and IWEX): at 750 m the agreement between data and models is very good, with IWEX being slightly better. At 350 m several of the measured towed vertical coherence spectra are more complex than the spectra from the deeper tows, there are anomalously high coherences in a band from 0.7 to 2 cycles per kilometer that are not predictable by the models. We suggest this coherence bump may be evidence of Eckart resonance, i.e., modes tunneling between the two thermoclines into the region of low buoyancy frequency.
    Description: Prepared for the Office of Naval Research under Contract N00014-74-C-0262; NR 083-004 and for the National Science Foundation under Grant OCE 74-19608.
    Keywords: Internal waves
    Repository Name: Woods Hole Open Access Server
    Type: Technical Report
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  • 10
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    Observations of interaction between the internal wavefield and low-frequency flows in the North Atlantic (2018)
    Woods Hole Oceanographic Institution
    Details
    Publication Date: 2022-05-26
    Description: Also published as: Journal of Physical Oceanography 9 (1979): 489-517
    Description: A total of four moorings from POLYMODE array I and II were analyzed in an investigation of the interaction of wavefields and mean flow. In particular, evidence for internal wave-mean flow interaction was sought by searching for time correlations between the vertically acting Reynolds stress of the wavefield (estimated using the temperature and velocity records), and the mean shear. No significant stress-shear correlations were found at the less energetic moorings (u¯≲10 cm s−1), indicating that the magnitude of the eddy viscosity was under 200 cm2 s−1, with the sign of the energy transfer uncertain. This is considerably below the O(4500 cm2 s−1) predicted by Müller (1976). An extensive error analysis indicates that the large wave stress predicted by the theory should have been observable clearly under the conditions of measurement. At moorings typified by a higher mean velocity (u¯≈25 cm s−1), statistically significant stress-shear correlations were found, and the wavefield energy level was observed to modulate with the strength of the mean shear. The observations were consistent with generation of short (∼1 km horizontal wavelength) internal waves by the mean shear near the thermocline, resulting in an effective eddy viscosity of ∼100 cm2 s−1. Theoretical computations indicate that the wavefield “basic state” may not be independent of the mean flow as assumed by Müller (1976) but can actually be modified by large-scale vertical shear and still remain in equilibrium. In that case, the wavefield does not exchange momentum with a large-scale vertical shear flow and, excepting critical-layer effects, a small vertical eddy viscosity is to be expected. Using the Garrett-Munk (1975) model internal wave spectrum, estimates were made of the maximum momentum flux (stress) expected to be lost to critical-layer absorption. This stress was found to increase almost linearly with the velocity difference across the shear zone, corresponding to a vertical eddy viscosity of −100 cm2 s−1. Stresses indicative of this effect were not observed in the data.
    Description: Prepared for the Office of Naval Research under Contract N00014-76-C-0197; NR 083-400.
    Keywords: Internal waves ; Ocean currents
    Repository Name: Woods Hole Open Access Server
    Type: Technical Report
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