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. dx.doi.org/10.1029/2011JC007681
In: Journal of Geophysical Research. American Geophysical Union: Richmond. ISSN 01480227; eISSN 21562202, meer
 
Auteurs   Top 
 King, B.
 Stone, M.
 Zhang, H.P.
 Gerkema, T., meer

 Marder, M.
 Scott, R.B.
 Swinney, H.L.


Abstract 
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 (M2) tidal frequency, omega(M2). Profiles of N2(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 N2(z) computed using Gibbs SeaWater routines in two thermodynamically equivalent expressions for N2(z) are found to yield values that are in excellent accord but differ significantly from N2(z) computed from oftenused but inexact expressions that involve potential density. Uncertainties in N2(z) are estimated using a Monte Carlo method, where the data are averaged over a range in depth (80200 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 (eastwest propagating) internal tides in the deep oceans. Inclusion of the full Coriolis effect (i.e., not making the socalled Traditional Approximation) leads to the conclusion that turning depths cannot occur for meridional and nearmeridional internal tides, but the 'nontraditional' component has little impact on turning depths for internal tides that are nearzonal (i.e., propagating within about 30 degrees of the eastwest direction) at low and midlatitudes. 
