Measurements and models of fine structure, internal gravity waves, and wave breaking in the deep Ocean

Abstract

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 E(a,w) = E()/No)-2(a/ao)-2 describes vertical current spectra and potential energy to horizontal kinetic energy ratios. The red wavenumber 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. 3Contours of temperature in depth versus time indicate possible mixing events. These events appear concurrently with high shear and Richardson numbers 0.25 < R < 1.0. Over 7 m a cutoff in R i 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.

Temporal Characteristics of Abyssal Finescale Motions above Rough Bathymetry

“…15) shows the characteristic peak at Ri Ͻ 1 and long tail of large Ri values. A scatterplot of N 2 versus S 2 (not shown) fails to clearly indicate the cutoff at Ri ϭ 1 ⁄4 that has been seen in other records (e.g., Eriksen 1978). Rather, the distribution is largely bound by the line N 2 ϭ 0.25 S 2 Ϫ 0.3 ϫ 10 Ϫ6 s…”

“…As opposed to Eriksen's (1978) example, the absence of a cutoff at Ri ϭ 1 ⁄4 might be because the 25-m current-meter separation distance was well within the range of the turbulent overturn heights. Kunze et al (1990) developed a parameterization for the turbulent dissipation and mixing due to shear instability in low Ri events.…”

“…In the following, the question of temperature uncertainty is set aside. Eriksen (1998), analyzing moored temperature data from the flank of Fieberling Guyot, related inversions to the turbulent mixing intensity through inferred changes in potential energy. Following Eriksen, relative to a final state in which the water column between two current meters (vertical distance of d) is homogenized in density, the available potential energy of a given overturn i per unit mass (P) is P ϭ Im (N .…”

Universality and strange attractors in internal-wave dynamics

“…X stratification is in the z direction, becomes greater than 2, one expects instability. The conventional Richardson number is j..l-2 , and there is quite striking evidence that internal wave modes with j..l > 2 (Ri<l/4) are absent in the oceanic internal wave field (Eriksen, 1978).…”

Double-diffusive processes : 1996 Summer Study Program in Geophysical Fluid Dynamics

“…Schmtt (1994) Expanded sce correlation e¡S diagrs from all Arctic-94 sttions (Figue 4) show tht the intrsion of new water occur in coherent, well-defined layers. Furer, the intrsions are algned in e¡S space, and appe imune to miing and disrption by shea flow and intern waves.…”