Experimental measurements of gas transfer under breaking and non-breaking waves

The wavy water surface and the turbulent flow beneath are essential to the transfer of momentum, energy, and mass between the atmosphere and the ocean. In a combined wind-water-wave tunnel we study the interactions between waves and turbulence in the water, focussing on the absorption of oxygen from the air. The waves are either wind-driven or mechanically generated, and the turbulence in the water bulk is excited by an active grid. This allows us to decouple turbulence and waves, resulting in a broad parameter space. The system is characterized in terms of surface properties by laser-induced fluorescence and a capacitive wave gauge, while the turbulence properties are measured by particle image velocimetry. In the absence of wind, bulk turbulence determines the gas transfer rate, as measured by an optical dipping probe. When waves are mechanically generated, turbulence still matters until the point at which wave steepness induces significantly non-linear behaviour. Under breaking waves, the effects of bulk turbulence are negligible, and we find much larger gas-transfer rates compared to non-breaking waves. We also discuss the effect of the finite fetch, and the role of bubble entrainment.