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Johannes R. Gemmrich and Hans van Haren

Abstract

Rapid temperature falls occurring at semidiurnal periods are observed close to the bottom above the continental slope in the Bay of Biscay. Simultaneous current measurements reveal that the abrupt temperature decrease O(0.5 K) within one minute is associated with a brief downslope current, contrary to previous observations. It is suggested that the flow field associated with internal waves propagating obliquely downslope is responsible for advecting denser water higher on the slope than lighter fluid, resulting in a gravitationally unstable stratification. The collapse of this stratification is observed as a thermal front passing the moored instruments.

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David M. Farmer and Johannes R. Gemmrich

Abstract

Measurements of temperature fluctuations and air entrainment within the upper 1 m of the surface caused by breaking surface waves have been acquired with a novel surface-following sensor array. Brief temperature fluctuations of order 20 mK–100 mK lasting up to 1.5 s and coincident with wave breaking are observed. Although a few of the temperature fluctuations represent direct measurements of the temperature of entrained air in large bubbles, it appears that in many cases the disruption of the ocean thermal boundary layer and its vertical mixing is observed. The observations suggest that wave breaking may play a role in the vertical mixing of warm or cool water stored in the top few centimeters near the ocean surface to depths at which more persistent structures continue the process. Only about 30% of whitecaps exhibit this temperature fluctuation. The authors hypothesize that measurable thermal injections tend to be concentrated in Langmuir convergence zones, which accentuate the thickness of the ocean thermal boundary layer.

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Johannes R. Gemmrich and David M. Farmer

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Ocean surface turbulence at high sea states is evaluated using heat as a naturally occurring passive tracer. A freely drifting instrument with a mechanically driven temperature profiler, fixed depth thermistors, and conductivity cells was used to monitor breaking wave activity and fine-scale temperature structure within the upper 2 m of the water column. The combination of temperature profiles and independent heat flux measurements demonstrate the presence of wave-enhanced turbulence and the effects of subsurface advection due to Langmuir circulation. The turbulence length scale, extracted from the temperature profile fine structure, suggests a surface value significantly smaller than previously reported. A Prandtl-type mixing length model matched with a surface energy flux due to wave breaking and the observed turbulent length scale is consistent with the authors’ observations. Both advection and enhanced diffusion are reconciled in a two-dimensional model of the upper-ocean boundary layer, providing a framework for studying Langmuir circulation and upper-ocean turbulence in terms of the measured temperature structure.

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Johannes R. Gemmrich and David M. Farmer

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Observations with a three-axis pulse-to-pulse coherent acoustic Doppler profiler and acoustic resonators reveal the turbulence and bubble field beneath breaking waves in the open ocean at wind speeds up to 14 m s−1. About 55%–80% of velocity wavenumber spectra, calculated with Hilbert spectral analysis based on empirical mode decomposition, are consistent with an inertial subrange. Time series of turbulent kinetic energy dissipation at approximately 1 m beneath the free surface and 1-Hz sampling rate are obtained. High turbulence levels with dissipation rates more than four orders larger than the background dissipation are linked to wave breaking. Initial dissipation levels beneath breaking waves yield the Hinze scale of the maximum bubble size a H ≅ 2 × 10−3 m. Turbulence induced by discrete breaking events was observed to decay as ε ∝ t n, where n = −4.3 is close to the theoretical value for isotropic turbulence (−17/4). In the crest region above the mean waterline, dissipation increases as ε(z) ∝ z 2.3. Depth-integrated dissipation in the crest region is more than 2 times the depth-integrated dissipation in the trough region. Adjusting the surface definition in common turbulence models to reflect the observed dissipation profile improves the agreement between modeled and observed dissipation. There is some evidence that turbulent dissipation increases above the background level prior to the air entrainment. The magnitude and occurrence of the prebreaking turbulence are consistent with wave–turbulence interaction in a rotational wave field.

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Michael Schwendeman, Jim Thomson, and Johannes R. Gemmrich

Abstract

Coupled in situ and remote sensing measurements of young, strongly forced wind waves are applied to assess the role of breaking in an evolving wave field. In situ measurements of turbulent energy dissipation from wave-following Surface Wave Instrument Float with Tracking (SWIFT) drifters and a tethered acoustic Doppler sonar system are consistent with wave evolution and wind input (as estimated using the radiative transfer equation). The Phillips breaking crest distribution Λ(c) is calculated using stabilized shipboard video recordings and the Fourier-based method of Thomson and Jessup, with minor modifications. The resulting Λ(c) are unimodal distributions centered around half of the phase speed of the dominant waves, consistent with several recent studies. Breaking rates from Λ(c) increase with slope, similar to in situ dissipation. However, comparison of the breaking rate estimates from the shipboard video recordings with the SWIFT video recordings show that the breaking rate is likely underestimated in the shipboard video when wave conditions are calmer and breaking crests are small. The breaking strength parameter b is calculated by comparison of the fifth moment of Λ(c) with the measured dissipation rates. Neglecting recordings with inconsistent breaking rates, the resulting b data do not display any clear trends and are in the range of other reported values. The Λ(c) distributions are compared with the Phillips equilibrium range prediction and previous laboratory and field studies, leading to the identification of several inconsistencies.

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Johannes R. Gemmrich and David M. Farmer

Abstract

Breaking of surface waves was monitored with conductivity measurements at wind speeds up to 18 m s−1. This method of wave breaking detection is well defined but excludes microbreakers and breaking of very short gravity waves. Observations in both fetch limited and open ocean conditions reveal that wind speed or wave age are insufficient to characterize breaking activity. A scaling of the breaking frequency based on wind energy input is proposed. This scaling collapses the authors’ diverse datasets, consistent with energy dissipation being determined primarily by the high frequency tail of the wave spectrum. Breaking waves with significant air entrainment were observed to have wavelengths between ∼0.1 of the dominant waves and that of the largest wind waves. The median value of the period of breaking waves is approximately half the period of the dominant waves and the mean height of breaking waves is ∼0.7 times the significant wave height. Less than 10% of observed breaking events resulted in deeply penetrating air entrainment (>0.2 m), suggesting the predominance of spilling breakers.

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Michael L. Banner, Johannes R. Gemmrich, and David M. Farmer

Abstract

Recent numerical model studies of nonlinear deep water wave group evolution suggest that wave breaking onset is associated primarily with a threshold behavior linked to the nonlinear wave group hydrodynamics. Motivated by these findings, a recently published probability analysis of observed dominant ocean wind wave breaking events reported a threshold behavior using the significant wave steepness as a measure of the mean nonlinearity of these waves. The present study investigates whether a similar threshold dependence in terms of an appropriate spectral measure of wave steepness, the spectral saturation, may be found for the breaking probability of shorter wind waves above the spectral peak. Extensive data records of open ocean whitecap breaking wave occurrences for wind speeds up to 18 m s−1 were analyzed for breaking probability dependence on spectral saturation in spectral bands with center frequencies ranging from 1 to 2.48 times the spectral peak frequency. Results are based on the measured ratio of passage rates past a fixed point of breaking crests to total crests for different wave scales. An extension of the zero-crossing method for counting wave crests was developed. Using this method the authors found that in any spectral subrange within the observed range of frequencies, a strong correlation exists between breaking probability and an appropriate mean spectral steepness parameter and that this correlation is characterized by a robust threshold behavior, just as was reported previously for the spectral peak waves. Further, to offset the influence of increasing directional spreading of the waves above the spectral peak frequency, an empirical directional spreading function was used to normalize the azimuth-integrated spectral saturation. Under this normalization, the spectral saturation threshold for breaking onset appears to have a common level over the frequency range investigated. This study also examined the correlation of breaking probability with spectral peak wave age. The low correlation found for all spectral ranges investigated suggests that nonlinear wave hydrodynamics are more important than wind forcing for the breaking of these wind waves.

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Michael L. Banner, Christopher J. Zappa, and Johannes R. Gemmrich

Abstract

There has been a recent upsurge in interest in quantifying kinematic, dynamic, and energetic properties of wave breaking in the open ocean, especially in severe sea states. The underpinning observational and modeling framework is provided by the seminal paper of O. M. Phillips. In this note, a fundamental issue contributing to the scatter in results between investigators is highlighted. This issue relates to the choice of the independent variable used in the expression for the spectral density of the mean breaking crest length per unit area. This note investigates the consequences of the different choices of independent variable presently used by various investigators for validating Phillips model predictions for the spectral density of the breaking crest length per unit area and the associated spectral breaking strength coefficient. These spectral measures have a central role in inferring the associated turbulent kinetic energy dissipation rate and the momentum flux to the upper ocean from breaking wave observations.

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Johannes R. Gemmrich, Michael L. Banner, and Chris Garrett

Abstract

Video observations of the ocean surface taken from aboard the Research Platform FLIP reveal the distribution of the along-crest length and propagation velocity of breaking wave crests that generate visible whitecaps. The key quantity assessed is Λ(c)dc, the average length of breaking crests per unit area propagating with speeds in the range (c, c + dc). Independent of the wave field development, Λ(c) is found to peak at intermediate wave scales and to drop off sharply at larger and smaller scales. In developing seas breakers occur at a wide range of scales corresponding to phase speeds from about 0.1 c p to c p, where c p is the phase speed of the waves at the spectral peak. However, in developed seas, breaking is hardly observed at scales corresponding to phase speeds greater than 0.5 c p. The phase speed of the most frequent breakers shifts from 0.4 c p to 0.2 c p as the wave field develops. The occurrence of breakers at a particular scale as well as the rate of surface turnover are well correlated with the wave saturation. The fourth and fifth moments of Λ(c) are used to estimate breaking-wave-supported momentum fluxes, energy dissipation rate, and the fraction of momentum flux supported by air-entraining breaking waves. No indication of a Kolmogorov-type wave energy cascade was found; that is, there is no evidence that the wave energy dissipation is dominated by small-scale waves. The proportionality factor b linking breaking crest distributions to the energy dissipation rate is found to be (7 ± 3) × 10−5, much smaller than previous estimates.

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