Contribution of the air entrainment to the gas transfer processes in wave-breaking events

Gas exchange processes at the air-sea interface play a crucial role in regulating the climate and sustaining human and marine life. It is known that a large portion of anthropogenic carbon dioxide is absorbed by the ocean, which, in turn, releases nearly half of the oxygen we breathe through the photosynthesis of marine flora in the sunlit upper ocean layer. 

Despite its relevance, the processes governing the gas transfer across the ocean surface are not fully understood. Although there is evidence that the bubbles generated by the wave breaking enhances significantly the gas transfer rate, in particular for low-solubility species, the parameterization of their contribution is inaccurate.

To investigate the phenomenon, the gas transfer occurring at the free surface of progressive waves is simulated by using high-fidelity simulations. A multiphase flow solver is employed to model the gas flux across the air-water interface and the diffusion processes in the air and water domains, making available data with a level of detail unattainable in experiments. Waves of different initial steepness leading to regular wave patterns, mild spilling, and intense plunging breakers are examined and comparisons in terms of the gas flux across the interface and the gas concentration in the two fluids are established. 

It is shown that the amount of gas transferred from the air to the water domain increases remarkably when wave breaking occurs, particularly in the presence of bubble entrainment. The availability of such detailed information allows us to compute the gas transfer velocity. Critical in this respect is the availability of the air-water interface actual area, a quantity generally unavailable in experiments. The increase in the gas transfer velocity is higher than the increase in the interface area across which the exchange takes place, meaning that there is an additional effect related to the enhanced turbulence associated with the bubble entrainment and the subsequent fragmentation process. It is also observed that provided the actual air-water interface area is accounted for, the gas transfer velocity scales approximately as the one-fourth power of the dissipation rate of the energy content in water, consistently with previous theoretical predictions.