Hendry Ross MacRae

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Ross MacRae

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  • Thesis
    The generation, energetics and propagation of internal tides in the western North Atlantic Ocean
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1975-04) Hendry, Ross MacRae
    This thesis reports on an investigation into the structure, energetics and propagation of tidal frequency internal waves. Data from Site D, near the New England continental slope, Muir Seamount northeast of Bermuda, and the Mid-Ocean Dynamics Experiment in the deep Sargasso Sea were used. Site D, in the near-field of a near-critical semidiurnal generation region, shows variable tidal currents and a marked surface intensification of M2 energy at the southern Site, related to the beam-like nature of the internal tide. The M2 tide dominates the semidiurnal band, with about 3 times more energy than at adjacent frequencies at 1/15 cpd separation. There is a significant phase locking between the M2 baroclinic currents and the equilibrium tide, and evidence for southward propagation of internal wave energy, suggesting generation at the slope to the north. The M2 baroclinic energy density is about 40% as great as the total barotropic energy density, but the internal tides have more horizontal kinetic energy. A seaward energy flux of .6 x 106 erg/s cm in the first three baroclinic M2 modes is much less than the .2 x 1010 erg/s cm shoreward energy flux in the surface tide. Difficul ties in interpreting the measurements are ascribed to the near-singular generation case. The MODE-l semidiurnal internal tides are also dominated by the M2 frequency, with a 3-fold energy increase over adjacent frequencies at 1/15 cpd separation. MODE-l is far from any major source of internal tides, but the measurements are much less variable than those from Site D. The extensive temperature measurements defining the MODE-l M2 internal tide are significantly coherent (phase locked) with the equilibrium tide, with about 80% of the coherent energy deriving from the first baroclinic mode, typical thermocline displacements being 3 m. A horizontal wavenumber spectrum estimate for the first mode M2 displacement fluctuations gives a peak at 160 km wavelength, in excellent agreement with the theoretical dispersion relation. The coherent first mode propagates on a bearing of 125°T, with a horizontal energy flux of .3 x 108 erg/s cm. Use of the weaker S2 internal tide and the dispersive nature of oceanic internal waves yields an estimate of 700 km to a common semidiurnal source region. The inferred range and bearing are consistent with generation at the Blake Escarpment and the continental slope to the northwest of the experiment. In one special case current and temperature measurements are combined in a local demonstration of the first mode M2 propagation, and the less extensive current data gives estimates of the barotropic tidal currents. Mooring motion, measured by pressure recorders on the mooring lines, accounts for about 15% of the semidiurnal temperature variance, but it is incoherent with the equilibrium tide. Diurnal tides were examined at all three locations. At the MODE-1 site - near the critical latitude for diurnal period internal waves - the current and temperture fields are dominated by high mode, incoherent, inertial-character morions which mask the tidal currents. About 25% of the diurnal band temperature variance is related to mooring motion. Muir Seamount provides a clear example of diurnal period internal tides trapped to their source region north of the critical latitude. A simple analytical model is developed for the diurnal period flow adjustment in a seamount geometry. Site D shows some evidence for diurnal period internal tides, but most of the energy in the diurnal currents is not simply related to the tidal forcing. Diurnal barotropic currents measured at Site D are combined with currents on the New England shelf, showing that the diurnal tidal wave behaves as a Kelvin-Stokes mode trapped to the slope, propagating along the depth contours to the west. Some aspects of simple generation models are considered. The slope north of Site D is not at all well described as an abrupt step for the M2 generations problem, but a more realistic model of Baines (1974) predicts the coherent fields observed. But the relatively small energy conversion from the surface tide to internal modes suggests that the globally near-critical slope north of Site D is a poor generator of internal tides in the deep sea, although the local energy density is high. The step shelf generation model is well suited for the steep Blake Escarpment, and predicts a seaward energy flux of .4 x 108 erg/s cm in the first mode, comparable to the measurements at MODE-1. This confirms theoretical expectations that the first baroclinic mode is not significantly damped by turbulent diffusion after propagating through the 700 km of ocean between the generation region and the MODE-1 deep ocean site.