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  • 1
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Minimal absorption in the ocean allows low-frequency acoustic transmissions over long horizontal distances. Measuring the travel time of the sound then yields large-scale integrals of the (inverse) sound speed, which are tightly related to heat content. The most ambitious application of this ...
    Type of Medium: Electronic Resource
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  • 2
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    American Institute of Physics for the Acoustical Society of America
    In:  Journal of the Acoustical Society of America, 100 (2) . pp. 797-813.
    Publication Date: 2018-04-13
    Description: The recently introduced notion of peak arrivals [Athanassoulis and Skarsoulis, J. Acoust. Soc. Am. 97, 3575–3588 (1995)], defined as the significant local maxima of the arrival pattern, is studied here as a modeling basis for performing ocean tomography. Peak arrivals constitute direct theoretical counterparts of experimentally observed peaks, and offer a complete modeling of experimental observables, even in cases where ray or modal arrivals cannot be resolved. The coefficients of the resulting peak‐inversion system, relating travel‐time with sound‐speed perturbations, are explicitly calculated in the case of range‐independent environments using normal‐mode theory. To apply the peak‐inversion scheme to tomography the peak identification and tracking problem is examined from a statistical viewpoint; maximum‐likelihood and least‐square solutions are derived and discussed. The particular approach adopted treats the identification and tracking problem in close relation to the inversion procedure; all possibilities of associating observed peaks with background arrivals are examined via trial inversions, and the best peak identification is selected with respect to a least‐square criterion. The feasibility of peak tomography is subsequently demonstrated using first synthetic data and then measured data from the THETIS‐I experiment. In the synthetic case the performance of the overall scheme is found to be satisfactory both with noise‐free and noisy data. Furthermore, the identification, tracking, and inversion results using experimental acoustic data from THETIS‐I are in good agreement with independent field observations.
    Type: Article , PeerReviewed
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  • 3
    Publication Date: 2018-06-15
    Description: Open-ocean deep convection is a littleunderstood process occurring in winter in remote areas under hostile observation conditions, for example, in the Labrador and Greenland Seas and near the Antarctic continent. Deep convection is a crucial link in the “Great Ocean Conveyor Belt” [Broecker, 1991], transforming poleward flowing warm surface waters through atmosphere-oceaninteraction into cold equatorward flowing water masses. Understanding its physics, interannual variations, and role in the global thermohaline circulation is an important objective of climate change research. In convection regions, drastic changes in water mass properties and distribution occur on scales of 10–100 km. These changes occur quickly and are difficult to observe with conventional oceanographic techniques. Apart from observing the development of the deep-mixed patch of homogeneous water itself, processes of interest are convective plumes on scales 〈1 km and vertical velocities of several cm s−1 [Schott et al., 1994] that quickly mix water masses vertically, and instability processes at the rim of the convection region that expedite horizontal exchanges of convected and background water masses [e.g., Gascard, 1978].
    Type: Article , NonPeerReviewed
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  • 4
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    American Institute of Physics for the Acoustical Society of America
    In:  Journal of the Acoustical Society of America, 116 (2). pp. 790-798.
    Publication Date: 2018-04-13
    Description: Ocean acoustic tomography is used to obtain heat-content estimates for the western Mediterranean basin. Travel-time data from 13 tomography sections of the Thetis-2 experiment (January–October 1994) are analyzed with a matched-peak inversion approach. The underlying analysis involves the use of peak arrivals and nonlinear model relations between travel-time and sound-speed variations. Slice inversion results are combined with temperature covariance functions for the western Mediterranean to obtain heat-content estimates for the basin. These estimates compare favorably with ECMWF data over the nine-month period of the Thetis-2 experiment. Furthermore, estimates for the basin-average temperature of the western Mediterranean deep water are obtained. © 2004 Acoustical Society of America.
    Type: Article , PeerReviewed
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  • 5
    Publication Date: 2017-07-19
    Description: One-year long travel-time data from the second deployment period of the Labrador Sea acoustic tomography experiment are analyzed, using a relative-time matched-peak approach, in order to invert for the sound-speed field and simultaneously solve for an unknown travel-time offset. From the relative-time inversions oceanographic information in terms of vertically averaged temperatures are deduced, yielding satisfactory matching with respect to available independent observations. The estimated offsets can be attributed to differential clock drifts, showing a clear parabolic behaviour over the course of the experiment, reaching maximum deviations from linear clock drift corrections (end-point calibrations) of the order of 50 ms. By applying the estimated second-order corrections to the travel-time data, absolute-time matched-peak inversions can then be performed. The used matched-peak approach accounts for the non-linear behaviour of travel times, which is due to the seasonally variable acoustic propagation conditions in the probed region, and turns out to be an appropriate tool in dealing with unknown travel-time offsets.
    Type: Article , PeerReviewed
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  • 6
    Publication Date: 2017-02-27
    Description: The ability to monitor the heat content of oceans over long distances is becoming increasingly important for understanding the role of oceans in climate change, for determining the variability of the state of the oceans, for operational ocean observing systems, and for studying large-scale ocean processes such as water-mass formation. Although the properties of the upper layers of the ocean can be routinely measured on large scales by satellite remote sensing (providing altimetric and infrared data) and with expendable probes dropped from commercial vessels, the deep interior of the ocean is more difficult to monitor. Ocean acoustic tomography1 is a promising technique for such applications, as it has the potential to provide systematic, instantaneous and repeated measurements of the ocean interior over large parts of an ocean basin. Here we demonstrate the capability of this technique for measuring the heat content across an entire (albeit small) ocean basin—the western Mediterranean Sea.
    Type: Article , PeerReviewed
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  • 7
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    In:  [Invited talk] In: International Conference Underwater Acoustic Measurements: Technologies and Results, 29.06, Heraklion, Greece .
    Publication Date: 2012-02-23
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 8
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    American Meteorological Society
    In:  Journal of Atmospheric and Oceanic Technology, 17 . pp. 240-254.
    Publication Date: 2018-07-04
    Description: A new approach based on statistical estimation is proposed for the analysis of tomographic traveltime data in cases of significant nonlinear dependence of the traveltimes on the sound-speed variations. Traditional tomography schemes based on linear perturbative inversions about a single, a priori fixed background state cannot properly handle such cases since the linearized model relations will lead to considerable inversion errors, depending on the extent of nonlinearity. In contrast, the background state is considered here as a variable unknown quantity to be estimated from the traveltime data, simultaneously with the peak identification function and the sound-speed perturbation. Using the maximum likelihood approach and the Gaussian assumption, the statistical estimation problem reduces to a weighted least squares problem to be solved simultaneously for the three unknown quantities. A posteriori inversion-error estimates are derived accounting also for uncertainties in the background selection and the peak identification. The proposed method is applied to nine-month-long traveltime data from the Thetis-2 experiment, conducted from January to October 1994 in the Western Mediterranean Sea, where the variability of the ocean environment gives rise to significant nonlinear dependencies between sound-speed and traveltime variations. The recovered temporal variability and stratification compare well with independent XBT observations.
    Type: Article , PeerReviewed
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  • 9
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 10
    Publication Date: 1997-02-01
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Published by Springer Nature
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