Turbulent Diffusivity under High Winds from Acoustic Measurements of Bubbles

Abstract

Breaking surface waves generate layers of bubble clouds as air parcels entrain into the upper ocean through the action of turbulent motions. The turbulent diffusivity in the bubble cloud layer is investigated by combining measurements of surface winds, waves, bubble acoustic backscatter, currents, and hydrography. These measurements were made at water depths of 60–90 m on the shelf of the Gulf of Alaska near Kayak Island during late December 2012, a period when the ocean was experiencing winds and significant wave heights up to 22 m s−1 and 9 m, respectively. Vertical profiles of acoustic backscatter decayed exponentially from the wave surface with e-folding lengths of about 0.6 to 6 m, while the bubble penetration depths were about 3 to 30 m. Both e-folding lengths and bubble depths were highly correlated with surface wind and wave conditions. The turbulent diffusion coefficients, inferred from e-folding length and bubble depth, varied from about 0.01 to 0.4 m2 s−1. Analysis suggests that the turbulent diffusivity in the bubble layer can be parameterized as a function of the cube of the wind friction velocity with a proportionality coefficient that depends weakly on wave age. Furthermore, in the bubble layer, on average, the shear production of the turbulent kinetic energy estimated by the diffusion coefficients is a similar order of magnitude as the dissipation rate predicted by the wall boundary layer theory.

Long‐Term Statistics of Observed Bubble Depth Versus Modeled Wave Dissipation

“…The surface is detected below a threshold (here 25 dB, corresponding to the 99th percentile). The thickness of the bubble layer is defined relative to the sea surface ( ) as the depth where the backscattered signal drops below 50 dB (Wang et al, ). The signal must be above 50 dB continuously to the surface or else it is removed (as it potentially represents a biological signal).…”

Flow variability within the Alaska Coastal Current in winter

“…Over the deployment period, the region was impacted by over 15 storms with winds of 10 m s −1 or higher. During storms, waters in the BWE study area became well‐mixed due to very vigorous mixing as indicated by high near‐surface and near‐bottom eddy diffusivities (≥2 × 10 −3 m 2 s −1 ) [ Moum , ; Wang et al ., ], while current observations showed along‐shelf nearly barotropic subtidal flow of 40 cm s −1 or more throughout the water column. The fluctuations of the along‐shelf flow were generally forced by the pressure gradient, i.e., the primary forcing mechanism of the flow fluctuations was the cross‐shelf pressure gradient resulting mainly from the cross‐shelf surface‐elevation gradient.…”

“…Moreover, the buoyancy frequency (N) calculated from the density data from S2 was higher at depths <20 m and the largest N (0.025 s 21 ) was mainly found at 13 m during the restratification periods. N was generally <0.015 s 21 at depths greater than 20 m. [Wang et al, 2016]. Turbulent kinetic energy dissipation rates of 10 26 W kg 21 or higher and resulting eddy diffusivities 2 3 10 23 m 2 s 21 also revealed vigorous mixing 9-10 m above the bottom during storms [Moum, 2015].…”

Uncertainties associated with in situ high-frequency long-term observations of suspended particulate matter concentration using optical and acoustic sensors

“…Schwendeman and Thomson, 2015) and as a consequence the subsurface scattering is perturbed by the sea state (e.g. Klein, 2003;Downing, 2006;Puleo et al, 2006) from surface up to depth (in m) that approximately equals the wind speed (in m/s) squared divided by 15 (Wang et al, 2016) or in the vicinity of bubble plumes (Nauw et al, 2015). Indeed, the finest bubble population of typical size less than 50 µm in radius penetrates within the water column where it can remain in near-equilibrium suspension (e.g.…”

“…Randolph et al, 2014). Outside the surf zone, bubbles can be carried down in form of plumes or patches by convergent fronts or wind-induced down-welling to depths up to 30 m. The vertical distribution of wave bubbles decays exponentially with depth resulting in a similar decrease of the backscattering strength at lengths scales of 0.5 to 5 m (Wang et al, 2016). Expressed in decibels the backscattering intensity displays a linear decrease with depth (e.g.…”