The Effects of Ice and Currents on Wave-breaking Turbulence at the Ocean Surface
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Breaking waves are critical to the exchange of momentum, gasses, and heat between the atmosphere and ocean. In open water, these exchanges control the growth and decay of waves, and have implications for global heat and gas budgets. However, local geophysical properties can significantly alter these exchanges. At river inlets, strong currents influence swell that has grown over a long ocean fetch, dramatically increasing surface fluxes. The reverse effect is seen in the presence of ice, where ocean wave properties are decoupled from atmospheric forcing, decreasing exchanges across the air/sea interface. Here, measurements of ocean waves and near surface turbulence are presented to show the modification of surface boundary processes from ice and currents. Measurements from free drifting buoys at the Mouth of the Columbia River are used to evaluate wave breaking parameterizations, where breaking occurs in intermediate depths and in the presence of vertically sheared currents. Breaking waves were identified using images collected with cameras onboard the buoys, and the breaking activity is well-correlated with wave steepness. Vertical shear in the currents produces a frequency-dependent effective current that modifies the linear dispersion relation. Accounting for these sheared currents in the wavenumber spectrum is essential in calculating the correct wave steepness; without this, wave steepness can be over (under) estimated on opposing (following) currents by up to 20%. The observed bulk wave steepness values suggest a limiting value of 0.4. The observed fraction of breaking waves is in good agreement with several existing models, each based on wave steepness. Further, a semi-spectral model designed for all depth regimes also compares favorably with measured breaking fractions. In this model, the majority of wave breaking is predicted to occur in the higher frequency bands (i.e., short waves). There is a residual dependance on directional spreading, in which wave breaking decreases with increasing directional spread. Observations at the Columbia River Mouth are also used to investigate the source and vertical structure of turbulence in the surface boundary layer. Turbulent velocity data collected onboard SWIFT buoys were corrected for platform motions to estimate turbulent kinetic energy (TKE) and TKE dissipation rates. Both of these quantities are correlated with wave steepness, which has been previously shown to determine wave breaking within the same dataset. Estimates of the turbulent length scale increase linearly with distance from the free surface, and roughness lengths estimated from velocity statistics scale with significant wave height. The vertical decay of turbulence is consistent with a balance between vertical diffusion and dissipation. Below a critical depth, a power law scaling commonly applied in the literature works well to fit the data. Above this depth, an exponential scaling fits the data well. These results, which are in a surface-following reference frame, are reconciled with results from the literature in a fixed reference frame. A mapping between free-surface referenced and mean-surface reference coordinates suggests 30% of the TKE is dissipated above the mean sea surface. Lastly, wind, wave, turbulence, and ice measurements from the Arctic Marginal Ice Zone are used to evaluate the response of the ocean surface to a given wind stress, with a focus on the local wind input to waves and subsequent ocean surface turbulence. Observations are from the Beaufort Sea in the summer and early fall of 2014, with fractional ice cover of up to 50%. Observations showed strong damping and directional modification of short waves, which, in turn, decreased the wind energy input to waves. Near-surface turbulent dissipation rates were also greatly reduced in partial ice cover. The reductions in waves and turbulence were balanced, suggesting that a wind-wave equilibrium is maintained in the marginal ice zone, though at levels much less than in open water. These results suggest that air-sea interactions are suppressed in the marginal ice zone relative to open ocean conditions at a given wind forcing, and this may act as a feedback mechanism in expanding a persistent marginal ice zone throughout the Arctic.
- Civil engineering