Response of the upper ocean to wind, wave and buoyancy forcing

Date

2017-08-03

Authors

Polonichko, Vadim Dmitri

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Abstract

At high winds, turbulence in the ocean surface mixed layer is dominated by organized coherent structures in the form of counterrotating helical vortices known as Langmuir cells. While the dynamics of the ocean surface layer has been studied rather extensively at lower wind speeds, the detailed physics at higher winds has remained largely inaccessible because of limited sea-going operations and difficulty conducting in situ measurements at high sea states. In the present thesis new measurement techniques, based on acoustical remote sensing, are described. A freely drifting imaging sonar was employed, which allowed us to follow time-evolving features for an extended period of time. This imaging sonar extends the acoustical approach beyond fixed orientation sonars and covers a full 360° circle on the surface. The full circle capability turns out to be a key addition to the measurements: it allowed quantitative evaluation of the directional properties of Langmuir circulation surface structure. These new methods allow us to sample near-surface circulation and bubble distributions even in extreme conditions, and contribute to our understanding of small scale dynamics in the wind driven surface layer. Using vertical velocity measurements in the convergent regions of Langmuir circulation and a model scaling, we infer the effective viscosity relevant to cell generation. Matching velocity- and temperature-inferred turbulent viscosities we estimate the depth scale over which the wind-wave forcing is of most importance. The velocity-inferred viscosity compares favorably with the mean model viscosity values evaluated at approximately two significant wave heights below the surface. Combining the effective viscosity calculated at different depths with the observed Stokes drift and friction velocity we estimate Langmuir numbers La between 0.015 and 0.1. We observe evolving cell patterns at larger La (between 0.02 and 0.05), which indicates that higher viscosity values than previously assumed in the models may be relevant for Langmuir circulation dynamics. Acoustical observations of the orientation of surface bubble clouds and the directional wave field during several deployments provided an opportunity for comparison of the directional properties of Langmuir circulation with a model that takes into account effects associated with misalignment of the Stokes drift and wind forcing. Model results imply that the growth rate is maximal overall when wind and waves are aligned. For a given angle between the Stokes drift and the wind (the misalignment angle) the direction of the cell axis for maximal growth lies between the Stokes drift and the wind and is mainly determined by (i) the misalignment angle and (ii) the ratio of the Stokes drift shear and mean Eulerian shear. Our ocean observations showed Langmuir cells responding to the changes in wind direction within 15 to 20 min. On two occasions, when the wind changed direction and waves lagged behind, the cells were observed to form in an intermediate direction (between wind and waves) consistent with model predictions. Observations of the near-surface circulation and thermal structure during a storm motivate analysis in terms of the Froude number derived from the measured vertical density gradient, the turbulent diffusivity which is inferred from the measured temperature distributions, and velocity and spatial structure of the circulation. The results demonstrate inhibition of Langmuir circulation by the presence of warm surface water at the beginning of a storm and provide a test of model description of the balance between wind-driven stirring and buoyant resistance. To better understand our measurements and the limitations of the approach, based on the acoustical backscatter, a technique for scatter location estimation is proposed. By comparing velocity magnitudes, independently measured with side-looking and upward-looking sonars, we estimate an effective scattering depth. These results show that the backscatter measured with side-looking sonars originates not right at the surface but at some depth below.

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Keywords

Plasma turbulence, Plasma frequencies, Turbulence, Plasma waves

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