Estimates of the coherence of ocean currents over vertical distances

844

458

We present a revised model for the distribution of internal wave energy in wave number frequency space. The model is empirical, guided by the following measurements: moored spectra and moored coherences for horizontal and vertical separations (MS, MHC, MVC as functions of frequency), towed spectra and towed vertical and time‐lagged coherences (TS, TVC, TLC as functions of horizontal wave number), and dropped spectra and dropped horizontal and lagged coherences (DS, DHC, DLC as functions of vertical wave number). Measurements are available for all but TLC and DHC. There is some indication of universality, suggesting perhaps a saturation limit.

Section: Moored Spectramentioning

confidence: 99%

Space-time scales of internal waves: A progress report

Garrett¹,

Munk²

1975

844

458

“…A major shortcoming of our model at frequencies near N is illustrated in Figure 4.9, where horizontal current half-coherence frequency is plotted from two MSA data sets and from Webster (1972). Note that both .the 2.8 m and 9.3 m vertical separations also include horizontal The simple model for horizontal current fine-structure seems to roughly describe the measured spectrum and first-difference spectra, but fails to describe coherence in a crucial.…”

mentioning

confidence: 93%

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

Eriksen¹

1978

110

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.

“…Nevertheless, their results of linearly decreasing coherence with vertical separation compared reasonably well with other, Eulerian data. In addition, following a proposal by Webster [1972] and observations by Siedler [1974], vertical separation distance Δ z , and thus coherence, was assumed to vary like Δ z ∼ σ −1 with frequency σ . However, Wunsch and Webb [1979] remarked that “many coherences are not monotonically decreasing functions of frequency”, albeit without specifying to what extent.…”

Section: Introductionmentioning

confidence: 99%

High‐resolution open‐ocean temperature spectra

Haren

Gostiaux

2009

[1] Accurate (<1 mK) temperature sensors have been stiffly moored at $1450 m in the open Canary Basin for 1.5 years while sampling at 1 Hz. The sensors were in an area where regular density steps occur. In this article, we investigate the variability of internal waves in such ''steppy'' environment. The waves vertically move layers of persistent temperature gradients for particular temperatures. The frequency (s) spectra of temperature, and more clearly those of inferred vertical currents w, show an internal wave band IWB that extends from 0.97f < s < N t , where f denotes the inertial frequency or vertical Coriolis parameter. Transition frequency N t is higher than buoyancy frequency N, computed over large vertical scales Dz = O(100) m. The extension of IWB beyond the traditional bounds [f,N] is probably due to small-vertical -scale layering (Dz % 1 m). The associated weak stratification between such thin layers provides small-scale N s % 4f h , where f h denotes the horizontal Coriolis parameter. This minimum stable stratification N s is associated with tilting of vorticity away from gravity, which causes the subinertial spectral extent to 0.97f. Isothermal smoothing reveals details of coherent vertical internal wave and incoherent motions. The present w spectrum continuum of coherent IWB motions is not flat, as in previous near-surface observations, but linearly increases from s = f to a peak at s $ 0.8N. The coherence spectrum shows a weak, significant peak at approximately twice the local buoyancy frequency for 2.5 Dz 100 m. Instead of dominant mode 1, zero phase difference, observed in IWB across the 130 m of observations, these super-buoyancy motions show mode 2 dominance, p phase difference.