Including water-side convection when calculating air-sea gas transfer for gases of different solubility

Combination of surface water cooling and a deep ocean mixed layer generates convective eddies scaling with the depth of a mixed layer that enhances the efficiency of the air-sea gas transfer. This enhancement is explained by the convective eddies disturbing the molecular diffusion layer and inducing increased turbulent mixing in the water (Rutgersson and Smedman, 2010; Rutgersson et al 2011). The enhancement can be introduced into existing formulations for calculating the air-sea exchange of gases by using an additional resistance, due to large-scale convection acting in parallel with other processes. The additional resistance is expressed by the convective velocity scale of the water and the friction velocity and characterizes the relative role of surface shear and buoyancy forces.

This is particularly important for the low to moderate wind-speed regime, when spray and bubbles have less dominance on the efficiency of the transfer. The water-side convection, as well as other processes influences the transfer velocity, influences gases of different solubility differently.

Data used in the study are based on measurements with the Eddy-Covariance technique (EC) taken at the Östergarnsholm site in the Baltic Sea as well as at a Swedish lake site (lake Tämnaren). The accuracy of EC data in relation to other methods of determining fluxes will be discussed. Instrumental uncertainty for transfer velocity for carbon dioxide using the EC method is estimated to approximately 20% (Rutgersson et al, 2008); in addition there are methodological problems as well as representativity of the data.

Using a modeling framework developed for the predictive analysis of the functioning and dynamics of the Baltic Sea organic/inorganic carbon and oxygen systems we analyzed the sensitivity of transfer velocity on the carbon cycle in the model. Significant components of the Baltic Sea carbon and oxygen systems relates to the uptake/release to the atmosphere. Improving the description of the air-sea transfer velocity changes the distribution of the air-sea exchange in time and space.