Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September, 1976
Description:
Measurements of horizontal and vertical current by propeller cluster
current meters and temperature by thermistors mounted on a rigid array
8 m high and 20 m long moored in the oceanic main thermocline near
Bermuda are interpreted in terms of thermocline-trapped internal wave
modes in the presence of temperature and density fine-structure.
Two turning-point uniformly valid asymptotic solutions to the internal
wave equation are developed to describe the wave functions. Mode
decay beyond the turning point in depth or frequency produces a sharp
cutoff in vertical current spectra above the local buoyancy frequency
N(z). An internal wave wavenumber-frequency spectral model Ε(α,ω) =
E(ω/No)-2 (α./α0)-2 describes vertical current spectra and potential energy
to horizontal kinetic energy ratios. The red wavenumer shape suppresses
peaks in both these quantities at frequencies near N(z). The data are
consistent with time-averaged horizontal isotropy of the wave field. A
dip in the vertical current spectra at 0.5 cph not predicted by the model
appears related to the bottom slope.
Temperature fine-structure is modeled as a passive vertical field
advected by internal waves. Quasi-permanent fine-scale features of the
stratification and vertically small-scale internal waves are indistinguishable
in this study. The model of McKean (1974) is generalized to
include fine-structure fields specified by their vertical wavenumber
spectra as well as different Poisson-distributed layer models. Together
with the trapped internal wave model, moored temperature spectra, temperature
vertical difference spectra, and coherence over vertical separations
are described using a fine-structure vertical wavenumber spectrum
PT(k) =ATk-5/2 which agrees with other spectra made using vertical profiling
instruments in the range 0.1 to 1.0 cpm.
Horizontal current fine-structure is also modeled as a passive field
advected vertically by long internal waves. The model describes moored
horizontal current spectra (least successfully at frequencies near N(z))
and finite-difference vertical shear spectra.
Contours of temperature in depth versus time indicate possible mixing
events. These events appear concurrently with high shear and
Richardson numbers O. 25≤ R ≤ 1.0. Over 7 m a cutoff in Ri at 0.25 is
observed, indicating saturation of the internal wave spectrum. Spectra
of finite-difference approximations to shear and buoyancy frequency are
dominated by fine-structure contributions over nearly the whole internal
wave range, suggesting that breaking is enhanced by fine-structure.
Breaking appears equally likely at all frequencies in the internal wave
range.
Description:
This research was supported by Office of Naval Research contract N00014-67-0204-0047 and continuation contract NOOOl4-75-C-0291.
Keywords:
Ocean waves
;
Internal waves
;
Gravity waves
;
Ocean currents
;
Fine-structure constant
;
Knorr (Ship : 1970-) Cruise KN52
;
Eastward (Ship) Cruise
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
6274218 bytes
Format:
application/pdf
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