Observations of Turbulence in the Ocean Surface Boundary Layer: Energetics and Transport

Abstract

Observations of turbulent kinetic energy (TKE) dynamics in the ocean surface boundary layer are presented here and compared with results from previous observational, numerical, and analytic studies. As in previous studies, the dissipation rate of TKE is found to be higher in the wavy ocean surface boundary layer than it would be in a flow past a rigid boundary with similar stress and buoyancy forcing. Estimates of the terms in the turbulent kinetic energy equation indicate that, unlike in a flow past a rigid boundary, the dissipation rates cannot be balanced by local production terms, suggesting that the transport of TKE is important in the ocean surface boundary layer. A simple analytic model containing parameterizations of production, dissipation, and transport reproduces key features of the vertical profile of TKE, including enhancement near the surface. The effective turbulent diffusion coefficient for heat is larger than would be expected in a rigid-boundary boundary layer. This diffusion coefficient is predicted reasonably well by a model that contains the effects of shear production, buoyancy forcing, and transport of TKE (thought to be related to wave breaking). Neglect of buoyancy forcing or wave breaking in the parameterization results in poor predictions of turbulent diffusivity. Langmuir turbulence was detected concurrently with a fraction of the turbulence quantities reported here, but these times did not stand out as having significant differences from observations when Langmuir turbulence was not detected.

Gas transfer velocity in the presence of wave breaking

“…. This scaling has been supported by several studies under different environmental conditions Greenan et al, 2001;Feddersen et al, 2007;Jones and Monismith, 2008;Gerbi et al, 2009). Beneath the WAL, the turbulent transport from the WBL is negligible, and o recovers to the wall layer theory.…”

“…There have been studies suggesting that Terray's scaling only holds for the H s of wind waves in which the swell components were excluded (Greenan et al, 2001;Gerbi et al, 2009). However, the FOSCS results indicated that Terray's scaling is also valid for the H s of mixed waves.…”

“…The spectra above the peak display the characteristics of the inertial subrange, where S(f) scales with f . Gerbi et al (2009) suggested that the spectrum above wave band is elevated by unsteady advection due to the surface gravity waves. To analyse this issue, the empirical mode decomposition (Huang et al, 1998) was applied to decompose the time series of the vertical velocity into different modes.…”

“…Significant deviations from the wall layer theory could result from other dynamical processes such as breaking surface waves. Wave breaking has been found to enhance the near-surface turbulent dissipation rate by 1Á2 orders of magnitude compared to the wall-layer theory (Kitaigorodskii, 1983;Agrawal et al, 1992;Anis and Moum, 1995;Drennan et al, 1996;Terray et al, 1996;Greenan et al, 2001;Soloviev and Lukas, 2003;Gemmrich and Farmer, 2004;Stips et al, 2005;Feddersen et al, 2007;Jones and Monismith, 2008;Gerbi et al, 2009). However, because of the difficulty of directly measuring the turbulence induced by breaking waves, the relation of the near-surface turbulence to the wind or wave parameters is not well understood.…”

Observations of the Transfer of Energy and Momentum to the Oceanic Surface Boundary Layer beneath Breaking Waves

“…Mitsuyasu (1985) and Rapp and Melville (1990) have suggested that under strong forcing the majority of the air-sea momentum flux is transferred by breaking waves, but few observational studies have directly measured both the air-and waterside momentum fluxes under these conditions (Gerbi et al 2008).…”