Impact of Siberian observations on the optimization of surface CO₂ flux

Abstract. To investigate the effect of additional CO₂ observations in the Siberia region on the Asian and global surface CO₂ flux analyses, two experiments using different observation data sets were performed for 2000–2009. One experiment was conducted using a data set that includes additional observations of Siberian tower measurements (Japan-Russia Siberian Tall Tower Inland Observation Network: JR-STATION), and the other experiment was conducted using a data set without the above additional observations. The results show that the global balance of the sources and sinks of surface CO₂ fluxes was maintained for both experiments with and without the additional observations. While the magnitude of the optimized surface CO₂ flux uptake and flux uncertainty in Siberia decreased from −1.17 ± 0.93 to −0.77 ± 0.70 Pg C yr⁻¹, the magnitude of the optimized surface CO₂ flux uptake in the other regions (e.g., Europe) of the Northern Hemisphere (NH) land increased for the experiment with the additional observations, which affect the longitudinal distribution of the total NH sinks. This change was mostly caused by changes in the magnitudes of surface CO₂ flux in June and July. The observation impact measured by uncertainty reduction and self-sensitivity tests shows that additional observations provide useful information on the estimated surface CO₂ flux. The average uncertainty reduction of the conifer forest of Eurasian boreal (EB) is 29.1 % and the average self-sensitivities at the JR-STATION sites are approximately 60 % larger than those at the towers in North America. It is expected that the Siberian observations play an important role in estimating surface CO₂ flux in the NH land (e.g., Siberia and Europe) in the future.