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Buoyancy frequency profiles and internal semidiurnal tide turning depths in the oceans
King, B.; Stone, M.; Zhang, H.P.; Gerkema, T.; Marder, M.; Scott, R.B.; Swinney, H.L. (2012). Buoyancy frequency profiles and internal semidiurnal tide turning depths in the oceans. J. Geophys. Res. 117.
In: Journal of Geophysical Research. American Geophysical Union: Richmond. ISSN 0148-0227, more
Peer reviewed article  

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Authors  Top 
  • King, B.
  • Stone, M.
  • Zhang, H.P.
  • Gerkema, T., more
  • Marder, M.
  • Scott, R.B.
  • Swinney, H.L.

    We examine the possible existence of internal gravity wave "turning depths," depths below which the local buoyancy frequency N(z) becomes smaller than the wave frequency. At a turning depth, incident gravity waves reflect rather than reaching the ocean bottom as is generally assumed. Here we consider internal gravity waves at the lunar semidiurnal (M-2) tidal frequency, omega(M2). Profiles of N-2(z) (the quantity in the equations of motion) are computed using conductivity, temperature, and depth data obtained in the World Ocean Circulation Experiment (WOCE). Values for N-2(z) computed using Gibbs SeaWater routines in two thermodynamically equivalent expressions for N-2(z) are found to yield values that are in excellent accord but differ significantly from N-2(z) computed from often-used but inexact expressions that involve potential density. Uncertainties in N-2(z) are estimated using a Monte Carlo method, where the data are averaged over a range in depth (80-200 m), which is determined by minimizing a cost function. Our principal result, reached from an analysis of all 18,000 WOCE casts, is that turning depths are common for zonal (east-west propagating) internal tides in the deep oceans. Inclusion of the full Coriolis effect (i.e., not making the so-called Traditional Approximation) leads to the conclusion that turning depths cannot occur for meridional and near-meridional internal tides, but the 'non-traditional' component has little impact on turning depths for internal tides that are near-zonal (i.e., propagating within about 30 degrees of the east-west direction) at low and midlatitudes.

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