Tellus: Series B, Chemical and Physical Meteorology (Jan 2017)
The influence of short-term variability in surface water on modelled air–sea exchange
Abstract
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 p$ \text{ CO}_2 $ on the annual air–sea $ \text{ CO}_2 $ 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 p$ \text{ CO}_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 p$ \text{ CO}_2 $ were obtained from a mixed layer scheme constrained by surface measurements of p$ \text{ CO}_2 $ (JENA). Short-term variability in surface water p$ \text{ CO}_2 $ was assessed by calculating monthly mean diurnal cycles from continuous measurements of surface water p$ \text{ CO}_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 $ \upmu $atm) were found from April to October. The present study showed that the temporal resolution of surface water p$ \text{ CO}_2 $ played an influential role on the annual air–sea $ \text{ CO}_2 $ exchange for the coastal study region. Hence, annual estimates of $ \text{ CO}_2 $ exchanges are sensitive to variation on much shorter time scales, and this variability should be included for any model study investigating the exchange of $ \text{ CO}_2 $ across the air–sea interface. Furthermore, the choice of surface p$ \text{ CO}_2 $ maps also had a crucial influence on the simulated air–sea $ \text{ CO}_2 $ exchange.
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