The influence of short-term variability in surface water pCO 2 on modelled air-sea CO 2 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 pCO(2) 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 pCO(2) 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 pCO(2) were obtained from a mixed layer scheme constrained by surface measurements of pCO(2) (JENA). Short-term variability in surface water pCO(2) was assessed by calculating monthly mean diurnal cycles from continuous measurements of surface water pCO(2), observed at stationary sites within the Baltic Sea. No apparent diurnal cycle was evident in winter, but diurnal cycles (with amplitudes up to 27 mu atm) were found from April to October. The present study showed that the temporal resolution of surface water pCO(2) 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 pCO(2) maps also had a crucial influence on the simulated air-sea CO2 exchange.