The rôle of capillary waves in oceanic air/water gas exchange

Abstract

Recent experiments have demonstrated that millimeter-scale capillary waves can enhance the transport of CO₂ by almost 2 orders of magnitude for moderate wave slopes. These results are used to create a model for the relative contribution of capillary waves to the gas exchange coefficient. The model input is wind speed u, and the output is K_f the fractional contribution of a specific range of capillary waves to the total gas exchange coefficient. Wind speed data, obtained via satellite, are used as the model input to obtain globally averaged values forK_f. In spite of the enhancing effect which capillary waves provide in the laboratory, the maximum value of K_f predicted by the model is less than 10%, and global averages are less than 4%. The small values of K_f are primarily due to the small wave energies predicted by existing wave height spectra in the high wave number regime. The uncertainty in existing wave height spectra, and the importance of experimental validation of the high wave number regime is discussed. Some interesting aspects of capillary wave gas exchange are also expanded upon. Among these are the demonstration of a linear relationship between the capillary wave gas exchange coefficient and wavelength, and a peak in the contribution of capillary waves to gas exchange at a wavelength around λ= 3.6 mm.