Hogg Nelson G.

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Nelson G.

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  • Technical Report
    Observations of internal Kelvin waves trapped round Bermuda
    (Woods Hole Oceanographic Institution, 1981-04) Hogg, Nelson G.
    Observations of the vertical and horizontal structure of motions near Bermuda have been made with two long-term moored arrays, one relatively far from and the other close to the island. Although not coincident in time, both arrays see horizontally coherent motions at II frequency bands ranging in period from 405 to 9.8 h. Only a peak at 26.1 h appears to be significant in the autospectra and, on several grounds, this is identified with the fundamental island-trapped mode (vertically and azimuthally). Additional resonant subinertial periods are at roughly 45, 54 and 90 h and these are vertical modes 2. 3 or 4 and azimuthal modes I or 2 propagating clockwise. The superinertial modes have less internal consistency but appear to have higher order vertical and azimuthal structures and both senses of azimuthal phase propagation. The subinertial vertical structure is modal and can be rationalized with baroclinic wave dynamics on a sloping bottom by defining an effective bottom depth as some reasonable average over the offshore decay scale. The subinertial motions are coherent with the surface wind stress and the phase between this forcing and the response changes by 180° across the trapped wave frequency bands consistent with a resonant system. The Q of the 26.1 h peak is at least 20 implying that dissipation has only a slight influence on the dynamics.
  • Technical Report
    A dynamical interpretation of low frequency motions near very rough topography : the Charlie Gibbs Fracture Zone
    (Woods Hole Oceanographic Institution, 1981-04) Hogg, Nelson G. ; Schmitz, William J. ;
    As a sequel to Schmitz and Hogg (1978), nine-month moored observations of current and temperature from the Charlie Gibbs Fracture Zone are further described, and then interpreted in terms of low frequency quasigeostrophic motions. Large vertical and horizontal changes are observed in the variance of both mean and fluctuating components. It is demonstrated that these changes could be associated with the (complex) nature of the topography. With regard to the mean flow, it is shown through an advective model that the topography is sufficiently steep to force this motion to closely follow isobaths. Time-dependent motions for periods from 2 to 96 days are described using the technique of empirical orthogonal functions. The most energetic mode is always bottom trapped, with flow oriented along isobaths at lower frequencies and approaching equipartition of along- and cross-isobath motions at higher frequencies. At the lowest frequencies a second mode which increases upward in energy is also judged significant, while for periods shorter than 3.6 days the second mode is again highly bottom trapped. We interpret these motions using linear wave theory. There is relatively close correspondence between theory and observation when the effects of both large- and small-scale topographic features are included in the model calculations. In addition to the usual topographic wave, the abrupt slope changes on the north wall allow for a baroclinic fringe mode with a ncar bottom node at low frequencies and small-scale bottom corrugations force highly bottom trapped waves above the smooth slope cut-off frequency.