The Impact of Transport Model Differences on CO2 Surface Flux Estimates from OCO-2 Retrievals of Column Average CO2.

We estimate the uncertainty of CO₂ flux estimates in atmospheric inversions stemming from differences between different global transport models. Using a set of Observing System Simulation Experiments (OSSEs), we estimate this uncertainty as represented by the spread between five different state-of-the-art global transport models (ACTM, LMDZ, GEOS-Chem, PCTM and TM5), for both traditional in situ CO₂ inversions as well as inversions of XCO₂ estimates from the Orbiting Carbon Observatory 2 (OCO-2). We find that in the absence of relative biases between in situ CO₂ and OCO-2 XCO₂, XCO₂-based estimates of terrestrial flux for TRANSCOM-scale land regions are more robust to transport model differences compared to corresponding in situ CO₂ inversions. This, however, does not hold for oceanic fluxes or flux estimates for zonal bands. We also find that the transport-driven uncertainty in fluxes is comparable between well-sampled northern temperate regions and poorly sampled tropical regions. Furthermore, we find that spatiotemporal differences in sampling, such as between OCO-2 land and ocean soundings, coupled with imperfect transport, can produce differences in flux estimates that are larger than flux uncertainties due to transport model differences. This highlights the need for sampling with as complete a spatial and temporal coverage as possible (e.g., using both land and ocean retrievals together for OCO-2) to minimize the impact of selective sampling. Finally, our annual and monthly estimates of transport-driven uncertainties can be used to evaluate the robustness of conclusions drawn from real OCO-2 and in situ CO₂ inversions. [ABSTRACT FROM AUTHOR]