Impact of stability on the gas transfer through the skin layer at the air-sea interface

The skin layer at the air-sea interface is dominating the transfer for substances with a high Schmidt number / small diffusion coefficient such as carbon dioxide. The stability may modify the mixing properties of the environment including the diffusive sublayer. In order to derive related parametrisations of the transfer velocity a conceptual framework is set up. The assumption of a fixed and aerodynamically smooth interface allows the construction of the gas transfer velocity depending on the friction velocity and the buoyancy flux in the weak wind limit. A wide range of field and laboratory data is used to determine four dimensionless tuning parameters. It is demonstrated how the stability changes the wind and Schmidt number dependence of the gas transfer velocity. Stable stratification may suppress wind-driven mixing until exclusively molecular diffusion is realising the transport. Free convection is the other limit which appears for unstable stratification. For passive tracers such as carbon dioxide a corresponding parameterisation of the transfer velocity is presented.