Wave breaking and sea state dependence of gas transfer velocities

Global air-sea gas fluxes estimates are based on parameterizations of the gas transfer velocity K. To first order, K is dictated by wind speed (U) and K is typically parameterized as a non-linear functions of U. There is however a large spread in K predicted by the traditional parameterizations at high wind speed. This is because a large variety of environmental forcing and processes (Wind, Currents, Rain, Waves, Breaking, Surfactants, Fetch) actually influence K and wind speed alone cannot capture the variability of air-water gas exchange.

At high wind speed especially, breaking waves become a key factor to take into account when estimating gas fluxes. Breaking results in additional upper ocean turbulence and generation of bubble clouds. Efforts have been made towards including the effect of bubble mediate transfer to reduce the uncertainties around K estimates at high wind speed. These parameterizations model the transfer velocity due to breaking as a function of fractional whitecap coverage (W) or windsea Reynolds numbers.

Here we will test existing parameterizations and explore ways of better constraining K based on whitecap coverage, sea state and breaking statistics. Both very soluble (such as Methanol and Acetone) and less soluble gasses (such as CO2, DMS) will be considered allowing to contrast the degree of wave breaking mediated transfer. The data used were collected during the HiWinGS (High Wind Gas exchange Study) cruise that took place in the North Atlantic during October and November 2013. Whitecap coverage and breaking statistics are determined from visible imagery acquired from the port and starboard side of the flying bridge of the R/V Knorr at 20Hz. Sea state conditions are computed from laser altimeter and wave rider buoy measurements. Eddy covariance fluxes and sea water concentration of CO2, DMS, Methanol and Acetone allow for direct calculation of transfer velocities.