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John A. Goff – One of the best experts on this subject based on the ideXlab platform.

  • Impact of synthetic Abyssal Hill roughness on resolved motions in numerical global ocean tide models
    Ocean Modelling, 2017
    Co-Authors: Patrick G. Timko, John A. Goff, Angelique Melet, Brian K. Arbic, Walter H. F. Smith, Joseph K. Ansong, Alan J. Wallcraft

    Abstract:

    Abstract Global models of seafloor topography have incomplete and inconsistent resolution at horizontal wavelengths less than about 10–20 km, notably due to their inability to resolve Abyssal Hills in areas unsurveyed by ships (that is, about 90% of the global seafloor). We investigated the impact of this unresolved bottom roughness on global numerical simulations of the HYbrid Coordinate Ocean Model (HYCOM) that are forced exclusively by the M2 and K1 internal tides. Simulations were run with horizontal resolutions of 0.08° and 0.04°, 10 isopycnal layers in the vertical direction, and two versions of bathymetry: one derived from the SRTM30_PLUS global bathymetry model, and one merging SRTM30_PLUS with a synthetic fractal surface simulating the expected roughness of Abyssal Hills in the 2–10 km horizontal wavelength band. Power spectra of the two bathymetry versions diverge at wavenumbers of order 4*10–4 radians/m and higher (wavelengths of order 15 km and lower), with more pronounced differences evident on the 0.04° grid, as the 0.08° grid has a more limited ability to capture bathymetric details at the Abyssal Hill scale. Our simulations show an increase in the amount of kinetic and potential energy in higher vertical modes, especially in the 0.04° simulation, when the synthetic roughness is added. Adding Abyssal Hills to the 0.04° simulation increases the M2 kinetic energy for modes 3 and 4 by 12–18% and the potential energy by 5 – 15%. Adding Abyssal Hills to the 0.08° simulation yields a reduced, though still measurable, impact on simulated baroclinic tidal energies. Baroclinic tidal energy conversion rates increase by up to 16% in regions of high roughness, and by up to 3.4% in the global integral. The 3.4% increase in global conversion rates in the numerical simulations is less than the 10% increase computed from a linear analysis on a 0.008° grid because of the resolution limitations of the numerical simulations. The results obtained in the present study, though limited by the horizontal and vertical resolutions of the simulations, are consistent with those of previous studies indicating that Abyssal Hills on the seafloor transfer energy into higher vertical mode internal tides. The method employed here to add synthetic roughness could easily be replicated in other models, with higher resolution and/or more complex forcing.

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  • LONG-TERM GOALS
    , 2016
    Co-Authors: John A. Goff, Brian K. Arbic

    Abstract:

    The small-scale roughness properties of the seafloor are increasingly being recognized as critical parameters in determining important processes in physical oceanography. For instance, in situ observations (e.g., Polzin et al., 1997) find that mixing levels are greatly elevated in regions of rough topography. Gille et al. (2000) demonstrate that mesoscale eddy energy tends to be lower in areas where the bottom is rough (suggesting the possibility that dissipation of eddy energy takes place in such areas), and Egbert and Ray (2003) show that substantial tidal dissipation occurs in such areas. The dissipation is generally thought to arise from the breaking of internal waves generated by flows over the rough seafloor. On the time scales of internal waves, mesoscale eddies and the general circulation can be regarded as steady, while tides are oscillatory. The physics of linear internal wave generation is different for these two classes of motions (e.g., Bell 1975), but for both types of flows the wave generation is strongly dependent on the horizontal and vertical scales inherent in the bottom topography. Using the classical formulation for lee waves (e.g., Cushman-Roisin, 1994, St. Laurent, 1999), one can argue that horizontal wavelengths ranging from ~60 m to 6 km generate internal waves when forced by steady flows. Features typical of Abyssal Hill morphology (e.g., 50 m height over 1 km horizontal scale) will generate a significant vertical internal wave energy flux. High-resolution regiona

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  • comment on glacial cycles drive variations in the production of oceanic crust
    Science, 2015
    Co-Authors: John A. Goff

    Abstract:

    Crowley et al. (Reports, 13 March 2015, p. 1237) propose that Abyssal Hill topography can be generated by variations in volcanism at mid-ocean ridges modulated by Milankovitch cycle–driven changes in sea level. Published values for Abyssal Hill characteristic widths versus spreading rate do not generally support this hypothesis. I argue that Abyssal Hills are primarily fault-generated rather than volcanically generated features.

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Angelique Melet – One of the best experts on this subject based on the ideXlab platform.

  • Impact of synthetic Abyssal Hill roughness on resolved motions in numerical global ocean tide models
    Ocean Modelling, 2017
    Co-Authors: Patrick G. Timko, John A. Goff, Angelique Melet, Brian K. Arbic, Walter H. F. Smith, Joseph K. Ansong, Alan J. Wallcraft

    Abstract:

    Abstract Global models of seafloor topography have incomplete and inconsistent resolution at horizontal wavelengths less than about 10–20 km, notably due to their inability to resolve Abyssal Hills in areas unsurveyed by ships (that is, about 90% of the global seafloor). We investigated the impact of this unresolved bottom roughness on global numerical simulations of the HYbrid Coordinate Ocean Model (HYCOM) that are forced exclusively by the M2 and K1 internal tides. Simulations were run with horizontal resolutions of 0.08° and 0.04°, 10 isopycnal layers in the vertical direction, and two versions of bathymetry: one derived from the SRTM30_PLUS global bathymetry model, and one merging SRTM30_PLUS with a synthetic fractal surface simulating the expected roughness of Abyssal Hills in the 2–10 km horizontal wavelength band. Power spectra of the two bathymetry versions diverge at wavenumbers of order 4*10–4 radians/m and higher (wavelengths of order 15 km and lower), with more pronounced differences evident on the 0.04° grid, as the 0.08° grid has a more limited ability to capture bathymetric details at the Abyssal Hill scale. Our simulations show an increase in the amount of kinetic and potential energy in higher vertical modes, especially in the 0.04° simulation, when the synthetic roughness is added. Adding Abyssal Hills to the 0.04° simulation increases the M2 kinetic energy for modes 3 and 4 by 12–18% and the potential energy by 5 – 15%. Adding Abyssal Hills to the 0.08° simulation yields a reduced, though still measurable, impact on simulated baroclinic tidal energies. Baroclinic tidal energy conversion rates increase by up to 16% in regions of high roughness, and by up to 3.4% in the global integral. The 3.4% increase in global conversion rates in the numerical simulations is less than the 10% increase computed from a linear analysis on a 0.008° grid because of the resolution limitations of the numerical simulations. The results obtained in the present study, though limited by the horizontal and vertical resolutions of the simulations, are consistent with those of previous studies indicating that Abyssal Hills on the seafloor transfer energy into higher vertical mode internal tides. The method employed here to add synthetic roughness could easily be replicated in other models, with higher resolution and/or more complex forcing.

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  • A three‐dimensional map of tidal dissipation over Abyssal Hills
    Journal of Geophysical Research: Oceans, 2015
    Co-Authors: Adrien Lefauve, Caroline Muller, Angelique Melet

    Abstract:

    The breaking of internal tides is believed to provide a large part of the power needed to mix the Abyssal ocean and sustain the meridional overturning circulation. Both the fraction of internal tide energy that is dissipated locally and the resulting vertical mixing distribution are crucial for the ocean state, but remain poorly quantified. Here we present a first worldwide estimate of mixing due to internal tides generated at small-scale Abyssal Hills. Our estimate is based on linear wave theory, a nonlinear parameterization for wave breaking and uses quasi-global small-scale Abyssal Hill bathymetry, stratification, and tidal data. We show that a large fraction of AbyssalHill generated internal tide energy is locally dissipated over mid-ocean ridges in the Southern Hemisphere. Significant dissipation occurs above ridge crests, and, upon rescaling by the local stratification, follows a monotonic exponential decay with height off the bottom, with a nonuniform decay scale. We however show that a substantial part of the dissipation occurs over the smoother flanks of mid-ocean ridges, and exhibits a middepth maximum due to the interplay of wave amplitude with stratification. We link the three-dimensional map of dissipation to Abyssal Hills characteristics, ocean stratification, and tidal forcing, and discuss its potential implementation in time-evolving parameterizations for global climate models. Current tidal parameterizations only account for waves generated at large-scale satellite-resolved bathymetry. Our results suggest that the presence of small-scale, mostly unresolved Abyssal Hills could significantly enhance the spatial inhomogeneity of tidal mixing, particularly above mid-ocean ridges in the Southern Hemisphere.

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  • A three-dimensional map of tidal dissipation over Abyssal Hills
    Journal of Geophysical Research. Oceans, 2015
    Co-Authors: Adrien Lefauve, Caroline Muller, Angelique Melet

    Abstract:

    The breaking of internal tides is believed to provide a large part of the power needed to mix the Abyssal ocean and sustain the meridional overturning circulation. Both the fraction of internal tide energy that is dissipated locally and the resulting vertical mixing distribution are crucial for the ocean state, but remain poorly quantified. Here we present a first worldwide estimate of mixing due to internal tides generated at small-scale Abyssal Hills. Our estimate is based on linear wave theory, a nonlinear parameterization for wave breaking and uses quasi-global small-scale Abyssal Hill bathymetry, stratification, and tidal data. We show that a large fraction of AbyssalHill generated internal tide energy is locally dissipated over mid-ocean ridges in the Southern Hemisphere. Significant dissipation occurs above ridge crests, and, upon rescaling by the local stratification, follows a monotonic exponential decay with height off the bottom, with a nonuniform decay scale. We however show that a substantial part of the dissipation occurs over the smoother flanks of mid-ocean ridges, and exhibits a middepth maximum due to the interplay of wave amplitude with stratification. We link the three-dimensional map of dissipation to Abyssal Hills characteristics, ocean stratification, and tidal forcing, and discuss its potential implementation in time-evolving parameterizations for global climate models. Current tidal parameterizations only account for waves generated at large-scale satellite-resolved bathymetry. Our results suggest that the presence of small-scale, mostly unresolved Abyssal Hills could significantly enhance the spatial inhomogeneity of tidal mixing, particularly above mid-ocean ridges in the Southern Hemisphere.

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Robert A. Pockalny – One of the best experts on this subject based on the ideXlab platform.

  • Bathymetric gradients of lineated Abyssal Hills: Inferring seafloor spreading vectors and a new model for Hills formed at ultra-fast rates
    Earth and Planetary Science Letters, 2006
    Co-Authors: Kelly A. Kriner, Robert A. Pockalny, Roger L. Larson

    Abstract:

    Abstract Abyssal Hill morphology provides a preliminary measure of the direction and rate of seafloor spreading, however, additional information (e.g., magnetic anomaly data or a nearby mid-ocean ridge) is usually required to verify these estimates. Previous attempts to identify a unique spreading rate proxy from Abyssal Hill dimensions (e.g., height, length, width) have largely failed due to the relatively large scatter of data or the non-linear character of spreading rate trends. We present a new, stand-alone method of determining both spreading rate and spreading direction using the distribution of azimuths for slopes facing toward and away from the ridge axis. The spreading rate is determined with the Δ peak height parameter, defined as the difference in the height (maximum frequency) of the two dominant modes observed in the azimuthal histograms. This parameter exhibits a clear, nearly linear spreading rate trend and allows half spreading rates to be estimated to within 10–20 km/Myr. The spreading direction is determined with the Δ peak width parameter, which compares the average width of the two dominant modes in the azimuthal histograms. The wider distribution of slope azimuths is oriented away form the paleo-ridge axis for all spreading rates, and thus spreading direction can be determined. The trends in the peak height and width parameters are used to constrain a new model of Abyssal Hill formation at ultra-fast spreading rates, which require greater off-axis extensional faulting resulting in a few large-throw faults on the outward-facing Hillsides, and many smaller throw faults on the inward-facing Hillsides.

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  • Tectonic evolution of the Pacific Phoenix Farallon triple junction in the South Pacific Ocean
    Earth and Planetary Science Letters, 2005
    Co-Authors: Richard F. Viso, Roger L. Larson, Robert A. Pockalny

    Abstract:

    Analysis of multibeam and gravity data reveals the tectonic history of the mid-Cretaceous (119–107 Ma) Penrhyn basin in the equatorial south Pacific Ocean. The basin formed during a period of heightened geodynamic activity and cessation of magnetic reversals. Similarities in the geometry of the Tongareva triple junction and the Rodriguez triple junction in the Indian Ocean make this study an interesting comparison between modern and ancient tectonics. Changes in Abyssal Hill trends during the formation of the basin suggest either a change in the location of the Euler pole describing the relative motion between the Pacific and Farallon plates, or a significant period of oblique spreading. Interaction between the local stress field associated with the break-up of the Manihiki plateau and the regional stress field controlling major plate motions complicated the tectonic evolution of the Penrhyn basin. Construction of velocity triangles from Abyssal Hill trends and measurements of the triple junction trace suggests that the triple junction oscillated between ridge–ridge–ridge and ridge–ridge–fault configurations. At least two reorganizations in the geometry of the triple junction occurred within 10 Ma of the initial rifting of the Manihiki plateau. Both changes in triple junction geometry coincide with discontinuities in the triple junction trace and result in right-lateral displacements of the triple junction trace. Changes in the bathymetric expression of the triple junction trace suggest a period of triple junction propagation controlled by rift propagation shortly after the change in Euler pole location.

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  • ANOMALOUSLY ROTATED Abyssal HillS ALONG ACTIVE TRANSFORMS : DISTRIBUTED DEFORMATION OF OCEANIC LITHOSPHERE
    Geology, 1999
    Co-Authors: Leslie J. Sonder, Robert A. Pockalny

    Abstract:

    Abyssal Hills within 20 km of the Clipperton and Pitman transforms deviate from regional trends by as much as 15°–20°. We use a thin viscous sheet model with power-law rheology to test the hypothesis that these anomalous trends result from distributed deformation of oceanic lithosphere, probably associated with periods of transpression. The variation of AbyssalHill trends with distance from the transform matches model predictions, with power-law exponents consistent with lithospheric rheology controlled by a combination of slip on faults at shallow depths and ductile creep in deeper parts. Predicted displacements are less than actual slip along the transforms, suggesting that only part of relative plate motion is taken up by distributed deformation. These results suggest that AbyssalHill rotation is sporadic, occurring perhaps when physical conditions (e.g., pore-fluid pressure, hydrothermal alteration, or transpression) are particularly favorable.

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