Analysis of a spectral wave model for air-sea carbon dioxide fluxes

Abstract

Gas transfer at the ocean-atmosphere boundary has a direct impact on global climate dynamics, gases budget cycling as well as several biological and environmental processes. This study focuses on the air-sea exchange of Carbon Dioxide (CO2) due to the role that it plays in ocean warming and marine life. The efficiency of the flux of CO2 across the boundary, known as the gas transfer velocity, kw, is directly controlled by the dynamics at the water surface. Many climate models use a kw formulation that depends mostly on the wind speed. However, field studies show that this relationship does not capture the data’s mean at high wind speeds and its overall variability. For an unbroken interface under stable conditions at low wind speeds, the transfer is mostly driven by slow conduction and molecular diffusion whereas under more turbulent conditions at higher wind speeds such as breaking waves, the gas transfer is mostly through air entrained through bubbles. The latter is much more difficult to constrain due to the complexity of several physical processes at different scales. Therefore, two recent sea-state dependent models are used along with an ocean-atmosphere coupled solver used to recreate wind-wave simulations for past cruises and their performances are analyzed with respect to the cruises’ measureddata. Both models capture the mean at high wind velocities and variability better than the wind-speed dependent model. A sensitivity analysis is also used to determine the impact of a few parameters that are defined by the models and propose changes that could help make the models better agree with the field data. Lastly, this study also looks at associating some of the variability observed in field data to spatial and temporal patterns in the simulation as well as non-linear gas transfer velocity-wind speed cycles observed in the models.