Nandi Papia

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  • Article
    Seismic reflection imaging of water mass boundaries in the Norwegian Sea
    (American Geophysical Union, 2004-12-14) Nandi, Papia ; Holbrook, W. Steven ; Pearse, Scott ; Paramo, Pedro ; Schmitt, Raymond W.
    Results from the first joint temperature and seismic reflection study of the ocean demonstrate that water mass boundaries can be acoustically mapped. Multichannel seismic profiles collected in the Norwegian Sea show reflections between the Norwegian Atlantic Current and Norwegian Sea Deep Water. The images were corroborated with a dense array of expendable bathythermographs and expendable conductivity-temperature depth profiles delineating sharp temperature gradients over vertical distances of ∼5–15 m at depths over which reflections occur. Fine structure from both thermohaline intrusions and internal wave strains is imaged. Low-amplitude acoustic reflections correspond to temperature changes as small as 0.03°C implying that seismic reflection methods can image even weak fine structure.
  • Article
    Seismic imaging of a thermohaline staircase in the western tropical North Atlantic
    (Copernicus Publications on behalf of the European Geosciences Union, 2010-07-02) Fer, Ilker ; Nandi, Papia ; Holbrook, W. Steven ; Schmitt, Raymond W. ; Paramo, Pedro
    Multichannel seismic data acquired in the Lesser Antilles in the western tropical North Atlantic indicate that the seismic reflection method has imaged an oceanic thermohaline staircase. Synthetic acoustic modeling using measured density and sound speed profiles corroborates inferences from the seismic data. In a small portion of the seismic image, laterally coherent, uniform layers are present at depths ranging from 550–700 m and have a separation of ~20 m, with thicknesses increasing with depth. The reflection coefficient, a measure of the acoustic impedance contrasts across these reflective interfaces, is one order of magnitude greater than background noise. Hydrography sampled in previous surveys suggests that the layers are a permanent feature of the region. Spectral analysis of layer horizons in the thermohaline staircase indicates that internal wave activity is anomalously low, suggesting weak internal wave-induced turbulence. Results from two independent measurements, the application of a finescale parameterization to observed high-resolution velocity profiles and direct measurements of turbulent dissipation rate, confirm these low levels of turbulence. The lack of internal wave-induced turbulence may allow for the maintenance of the staircase or may be due to suppression by the double-diffusive convection within the staircase. Our observations show the potential for seismic oceanography to contribute to an improved understanding of occurrence rates and the geographical distribution of thermohaline staircases, and should thereby improve estimates of vertical mixing rates ascribable to salt fingering in the global ocean.