Sensitivity of the air–sea CO₂ exchange in the Baltic Sea and Danish inner waters to atmospheric short-term variability

Abstract. Minimising the uncertainties in estimates of air–sea CO₂ exchange is an important step toward increasing the confidence in assessments of the CO₂ cycle. Using an atmospheric transport model makes it possible to investigate the direct impact of atmospheric parameters on the air–sea CO₂ flux along with its sensitivity to, for example, short-term temporal variability in wind speed, atmospheric mixing height and atmospheric CO₂ concentration. With this study, the importance of high spatiotemporal resolution of atmospheric parameters for the air–sea CO₂ flux is assessed for six sub-basins within the Baltic Sea and Danish inner waters. A new climatology of surface water partial pressure of CO₂ (pCO₂^w) has been developed for this coastal area based on available data from monitoring stations and on-board pCO₂^w measuring systems. Parameterisations depending on wind speed were applied for the transfer velocity to calculate the air–sea CO₂ flux. Two model simulations were conducted – one including short-term variability in atmospheric CO₂ (VAT), and one where it was not included (CAT).

A seasonal cycle in the air–sea CO₂ flux was found for both simulations for all sub-basins with uptake of CO₂ in summer and release of CO₂ to the atmosphere in winter. During the simulated period 2005–2010, the average annual net uptake of atmospheric CO₂ for the Baltic Sea, Danish straits and Kattegat was 287 and 471 Gg C yr⁻¹ for the VAT and CAT simulations, respectively. The obtained difference of 184 Gg C yr⁻¹ was found to be significant, and thus ignoring short-term variability in atmospheric CO₂ does have a sizeable effect on the air–sea CO₂ exchange. The combination of the atmospheric model and the new pCO₂^w fields has also made it possible to make an estimate of the marine part of the Danish CO₂ budget for the first time. A net annual uptake of 2613 Gg C yr⁻¹ was found for the Danish waters.

A large uncertainty is connected to the air–sea CO₂ flux in particular caused by the transfer velocity parameterisation and the applied pCO₂^w climatology. However, as a significant difference of 184 Gg C yr⁻¹ is obtained between the VAT and CAT simulations, the present study underlines the importance of including short-term variability in atmospheric CO₂ concentration in future model studies of the air–sea exchange in order to minimise the uncertainty.