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  • Articles  (276)
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  • 1
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    Efficiency of Turbulent Mixing in the Abyssal Ocean Circulation (2017)
    Wiley
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    Publication Date: 2017-05-03
    Description: Turbulent mixing produced by breaking of internal waves plays an important role in setting the patterns of downwelling and upwelling of deep dense waters and thereby helps sustain the global deep ocean overturning circulation. A key parameter used to characterize turbulent mixing is its efficiency, defined here as the fraction of the energy available to turbulence that is invested in mixing. Efficiency is conventionally approximated by a constant value near 1/6. Here we show that efficiency varies significantly in the abyssal ocean and can be as large as ∼1/3 in density stratified regions near topographic features. Our results indicate that variations in efficiency exert a first order control over the rate of overturning of the lower branch of the meridional overturning circulation.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Reconciling the ICE-6G C reconstruction of glacial chronology with ice-sheet dynamics: the cases of Greenland and Antarctica (2015)
    Wiley
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    Publication Date: 2015-08-25
    Description: We describe a theoretical and numerical framework that has been developed to investigate the compatibility of the ICE-6G_C reconstruction of the glaciation histories of the Greenland and Antarctic ice sheets with the latest understanding of ice physics. The ICE-6G_C reconstruction has been produced solely on the basis of the theory of the glacial isostatic adjustment (GIA) process and it has remained an issue as to whether such reconstructions of the time dependent thickness variations of grounded continental ice sheets were compatible with physics-based ice mechanical considerations. Our analyses focus on the evolution over the last glacial cycle of these extent ice sheet complexes and demonstrate that the GIA inferred models are entirely consistent with such considerations when uncertainties in (net) mass balance history are taken fully into account.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    The high-pressure electronic spin transition in iron: Potential impacts upon mantle mixing (2011)
    Wiley
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    Publication Date: 2011-08-09
    Description: At the Rayleigh number appropriate to Earth's mantle, radial heat transport is dominated by solid state thermal convection. Because of the large number of physical properties required to determine the Rayleigh number, and because these properties are expected to be (perhaps strong) functions of pressure and temperature (P-T), laboratory measurements of them under the high pressure and temperature conditions that occur in the deep Earth are of fundamental importance. Recent experimental data demonstrate that an electronic spin transition in iron that occurs at midmantle depths results in significant changes in the physical properties of the ferropericlase component of mantle mineralogy. Additional recent results suggest that it may also exist in the dominant perovskite component. Using control volume based numerical models we investigate the impacts on mantle mixing of this spin transition through its influence on the most important subset of these physical properties, namely density, thermal expansivity, bulk modulus and heat capacity. Our numerical model results demonstrate that this electronic transition enhances mixing in the lower regions of the lower mantle by enhancing the vigor of rising plumes. The lowermost region of the mantle is slightly warmed and the upper mantle slightly cooled by spin-induced effects. However, the spin crossover in the lower mantle appears not to significantly influence mantle layering. Due to the competition that could exist between the strength of the spin-induced thermodynamic properties of ferropericlase and perovskite, cold descending thermal anomalies could stagnate at middle-to-lower mantle depths and lead to the occurrence of “mid mantle avalanches.”
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    High-resolution numerical modeling of tides in the western Atlantic, Gulf of Mexico, and Caribbean Sea during the Holocene (2011)
    Wiley
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    Publication Date: 2011-10-14
    Description: Tidal constituents and datums are computed on a high resolution grid of the northwestern Atlantic Ocean, including the Gulf of Mexico and the Caribbean Sea. A global model is used to determine tidal parameters on a grid with a nominal resolution of 800 × 800. The global model includes self-attraction and loading, drag in shallow marginal seas, and internal tide drag in the deep ocean. Simulations are performed at 1000 year intervals during the Holocene (10,000 calibrated years before present (10 ka)) in combination with changes in bathymetry and coastline location derived from a glacial isostatic adjustment model. The global model results are then used to force a regional barotropic tidal model. The regional model uses an unstructured finite element grid, with very high resolution at the coastline. The model results reveal significant variations in tidal constituent amplitudes throughout the Holocene. In the northwestern Atlantic, semi-diurnal components show a strong amplification at around 9 ka while in the Gulf of Mexico, the response is much more muted. Variations in diurnal tidal parameters are found to be less significant than semi-diurnal parameters throughout the model domain. Changes in tidal range, of great relevance to changes in relative sea level (RSL), are also investigated throughout the Holocene. The overall structure is similar to the patterns observed in the M2 tide, with peak increases of 200–300%, relative to present-day, being observed along the east coast of the United States from 9 to 8 ka. Finally, the high spatial resolution of the regional model allows for the investigation of tidal changes at spatial scales (e.g., individual bays) much smaller than in previous studies.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 5
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    A high-resolution study of tides in the Delaware Bay: Past conditions and future scenarios (2013)
    Wiley
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    Publication Date: 2013-02-02
    Description: [1]  Tides in the Delaware Bay (USA) have been modeled from 7000 years before present (7 ka) to the present day and for selected future sea-level rise scenarios (100 years, 300 years). Historic bathymetries were constructed through use of glacial isostatic adjustment models and a very high spatial resolution (〈 100 m) was used at the shoreline. Future bathymetries were obtained by extrapolating these glacial isostatic adjustment models and applying an additional eustatic sea-level rise. It was found that tides in the lower bay have remained fairly constant through time but that tides in the upper bay have increased steadily from about 4 ka to the present day; a nearly 100% increase in total. The future runs demonstrated spatially complex behavior with tidal-range changes of up to 10%.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    GRACE era secular trends in Earth rotation parameters: A global scale impact of the global warming process? (2011)
    Wiley
    Details
    Publication Date: 2011-05-24
    Description: Recent trends in the two primary anomalies in the rotational state of the planet are analyzed in detail, namely those associated with the speed and direction of polar wander and with the non-tidal acceleration of the rate of axial rotation (via the measurement of the changing oblateness of the Earth's shape). It is demonstrated that a significant change in the secular trends in both of these independent parameters became evident subsequent to approximately 1992. It is suggested that both parameters might have come to be substantially influenced by mass loss from both the great polar ice sheets, and from the very large number of small ice-sheets and glaciers that are also being influenced by the global warming phenomenon. The modern values for the secular drifts in those parameters that we estimate are appropriate to the period during which measurements have been made by the satellites of the Gravity Recovery and Climate Experiment (GRACE). These changes in secular rates might greatly assist in understanding why the GRACE-inferred values of the time derivatives of the degree two and order one Stokes coefficients differ so significantly from those associated with Late Quaternary ice-age influence.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 7
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    Spatiotemporal development of irreversible mixing in mid-latitude baroclinic wave life cycles: morphology, energetics, and non-isentropic mixing activity (2014)
    Wiley
    Details
    Publication Date: 2014-02-28
    Description: [1]  The significance of non-isentropic irreversible mixing processes is diagnosed for idealized simulations of synoptic-scale baroclinic wave life-cycles along the subtropical jet stream, using a non-hydrostatic, anelastic, mesoscale model subject to a free-slip surface boundary condition. A variety of morphological features of mixing are identified such as a mesoscale columnar vortex associated with the onset of frontal fracture, episodic overturnings along the surface fronts, “wrinkling” of the tropopause and injection of tropospheric air into the stratosphere. The evolution of the degree of non-isentropic irreversible mixing is first analysed by computing the change of the “base” component of potential energy that cannot be converted into kinetic energy. The structure of the mixing activity is also diagnosed through inspection of spatiotemporal changes of entropy to demonstrate that the surface fronts are by far the most active regions of such activity. This activity is found to be primarily longitudinal, extending from the surface to the lower stratosphere, to form a three-dimensional spiral in the synoptic-scale cyclone and along the fronts. However, an exceptional region also exists along the warm front, where the structure becomes primarily transverse in the mature phase of frontal development in a model including an explicit representation of small-scale turbulence. In all simulations, the net transfer of mass and heat across the tropopause is from troposphere to stratosphere. The maximum transfer occurs when the observed climatological level of stratification contrast is assumed between stratosphere and troposphere. The same climatological choice leads to a minimum net irreversible mixing, which occurs primarily at Earth's surface.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 8
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    Turbulence transition in stratified atmospheric and oceanic shear flows: Reynolds and Prandtl number controls upon the mechanism (2011)
    Wiley
    Details
    Publication Date: 2011-08-30
    Description: We provide detailed analyses of the role of secondary instabilities in the transition to turbulence of stratified shear flows at the high Reynolds and Prandtl numbers appropriate to the atmosphere and ocean. Our focus is upon a series of recent observations suggestive of a marked change in the mechanism of transition in this laboratory inaccessible regime. Our results demonstrate the joint dependance of the transition mechanism on these two nondimensional parameters. We discuss the primary transition mechanisms of shear aligned convective instability, stagnation point instability and shear instabilities of the braid in detail.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 9
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    Comment on “Ocean mass from GRACE and glacial isostatic adjustment” by D. P. Chambers et al. (2012)
    Wiley
    Details
    Publication Date: 2012-11-03
    Description: The modern global theory of the glacial isostatic adjustment (GIA) process is a theory that directly addresses the extent to which the geoid of classical geodesy is impacted by this phenomenon. Because the geoid is, by conventional definition, the surface of constant potential that overlaps the surface of the sea in the absence of currents and tides, we may determine the impact of the GIA process upon it only by explicit analysis of the manner in which mass is redistributed among the ocean basins and the level of the sea is thereby influenced. Although the dominant contribution to GIA is that associated with the transfer of mass between the oceans and the continents, there is an additional influence due to the variations in Earth's rotational state. This influence “feeds-back” onto the geoid. In the recent paper by Chambers et al. (2010), several arguments were presented that question earlier attempts to discuss the consequences of this feedback. These arguments are interesting and we address them in what follows.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    Global sea level rise and the greenhouse effect: might they be connected? (1989)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1989-05-19
    Description: Secular sea level trends extracted from tide gauge records of appropriately long duration demonstrate that global sea level may be rising at a rate in excess of 1 millimeter per year. However, because global coverage of the oceans by the tide gauge network is highly nonuniform and the tide gauge data reveal considerable spatial variability, there has been a well-founded reluctance to interpret the observed secular sea level rise as representing a signal of global scale that might be related to the greenhouse effect. When the tide gauge data are filtered so as to remove the contribution of ongoing glacial isostatic adjustment to the local sea level trend at each location, then the individual tide gauge records reveal sharply reduced geographic scatter and suggest that there is a globally coherent signal of strength 2.4 +/- 0.90 millimeters per year that is active in the system. This signal could constitute an indication of global climate warming.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Peltier, W R -- Tushingham, A M -- New York, N.Y. -- Science. 1989 May 19;244(4906):806-10.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17802238" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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