The influence of short-term variability in surface water pCO2 on modelled air–sea CO2 exchange

Coastal seas and estuarine systems are highly variable in both time and space and with their heterogeneity difficult to capture with measurements. Models are useful tools in obtaining a better spatiotemporal coverage or, at least, a better understanding of the impacts such heterogeneity has in driving variability in coastal oceans and estuaries. A model-based sensitivity study is constructed in this study in order to examine the effects of short-term variability in surface water pCO₂ on the annual air–sea CO₂ exchange in coastal regions. An atmospheric transport model formed the basis of the modelling framework for the study of the Baltic Sea and the Danish inner waters. Several maps of surface water pCO₂ were employed in the modelling framework. While a monthly Baltic Sea climatology (BSC) had already been developed, the current study further extended this with the addition of an improved near-coastal climatology for the Danish inner waters. Furthermore, daily surface fields of pCO₂ were obtained from a mixed layer scheme constrained by surface measurements of pCO₂ (JENA). Short-term variability in surface water pCO₂ was assessed by calculating monthly mean diurnal cycles from continuous measurements of surface water pCO₂, observed at stationary sites within the Baltic Sea. No apparent diurnal cycle was evident in winter, but diurnal cycles (with amplitudes up to 27 μatm) were found from April to October. The present study showed that the temporal resolution of surface water pCO₂ played an influential role on the annual air–sea CO₂ exchange for the coastal study region. Hence, annual estimates of CO₂ exchanges are sensitive to variation on much shorter time scales, and this variability should be included for any model study investigating the exchange of CO₂ across the air–sea interface. Furthermore, the choice of surface pCO₂ maps also had a crucial influence on the simulated air–sea CO₂ exchange.