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

[摘要] Minimising the uncertainties in estimates of air–sea CO₂ exchange is animportant step toward increasing the confidence in assessments of theCO₂ cycle. Using an atmospheric transport model makes it possible toinvestigate the direct impact of atmospheric parameters on the air–seaCO₂ flux along with its sensitivity to, for example, short-term temporalvariability in wind speed, atmospheric mixing height and atmosphericCO₂ concentration. With this study, the importance of highspatiotemporal resolution of atmospheric parameters for the air–sea CO₂flux is assessed for six sub-basins within the Baltic Sea and Danish innerwaters. A new climatology of surface water partial pressure of CO₂(pCO₂^w) has been developed forthis coastal area based on available data from monitoring stations andon-board pCO₂^w measuringsystems. Parameterisations depending on wind speed were applied for thetransfer velocity to calculate the air–sea CO₂ flux. Two modelsimulations were conducted – one including short-term variability inatmospheric CO₂ (VAT), and one where it was not included (CAT).

A seasonal cycle in the air–sea CO₂ flux was found for both simulationsfor all sub-basins with uptake of CO₂ in summer and release of CO₂to the atmosphere in winter. During the simulated period 2005–2010, theaverage annual net uptake of atmospheric CO₂ for the Baltic Sea, Danishstraits and Kattegat was 287 and 471 Gg C yr⁻¹ for theVAT and CAT simulations, respectively. The obtained difference of 184 Gg C yr⁻¹ was found to be significant, and thus ignoring short-termvariability in atmospheric CO₂ does have a sizeable effect on theair–sea CO₂ exchange. The combination of the atmospheric model and thenew pCO₂^w fields has also madeit possible to make an estimate of the marine part of the Danish CO₂budget for the first time. A net annual uptake of 2613 Gg C yr⁻¹ wasfound for the Danish waters.

A large uncertainty is connected to the air–sea CO₂ flux in particularcaused by the transfer velocity parameterisation and the appliedpCO₂^w climatology. However, as asignificant difference of 184 Gg C yr⁻¹ is obtained between the VAT andCAT simulations, the present study underlines the importance of includingshort-term variability in atmospheric CO₂ concentration in futuremodel studies of the air–sea exchange in order to minimise the uncertainty.

[学科分类] 地球化学与岩石