The sensitivity of deduced CO₂ sources and sinks to variations in transport and imposed surface concentrations

Carbon dioxide measurements exhibit substantial interannual variability in the growth rate and spatial distribution of CO₂ in the atmosphere. Here we consider typical variations in surface spatial distribution and year to year variability in transport and deduce what changes are required to the source and sink distributions of CO₂ to account for these variations. We use a three-dimensional (3d) tracer transport model with imposed surface CO₂ concentrations to deduce the sources required for consistency with these concentrations. Experiments have been conducted in which the tracer model winds and prescribed surface concentrations were varied and the impact on the sources assessed. The results indicate that changes to the transport (even in an extreme ENSO year) have only a small impact on the derived sources compared with the impact resulting from changes in the imposed concentrations. This could imply that the observed year to year variability in the annual mean spatial distribution of CO₂ results from changes in the CO₂ source distributions rather than changes in the transport.