Regional CO 2 Fluxes during 2010–2015 Inferred from GOSAT XCO 2 retrievals using a new version of Global Carbon Assimilation System

Abstract. Satellite XCO2 retrievals could help to improve carbon flux estimation because of their good spatial coverage. In this study, to assimilate the GOSAT XCO2 retrievals, the Global Carbon Assimilation System (GCAS) is upgraded with new assimilation algorithms, procedures and a localization scheme, a higher assimilation parameter resolution and so on, and hence is named as GCASv2. Based on this new system, the global terrestrial ecosystem (BIO) and ocean (OCN) carbon fluxes from May 1, 2009 to Dec 31, 2015 are constrained using the GOSAT ACOS XCO2 retrievals (Version 7.3). The posterior carbon fluxes from 2010 to 2015 are independently evaluated using CO2 observations from 52 surface flask sites. The results show that the posterior carbon fluxes could significantly improve the modeling of atmospheric CO2 concentrations, with global mean BIAS decreases from a prior value of 1.6 ± 1.8 ppm to -0.5 ± 1.8 ppm. Globally, the mean annual BIO and OCN carbon sinks and their interannual variations inferred in this study are very close to the estimates of CT2017 during the study period, and the inferred mean atmospheric CO2 growth rate and its interannual changes are also very close to the observations. Regionally, over the northern lands, there are the strongest carbon sinks in North America Temperate, followed by Europe, Boreal Asia, and Temperate Asia; and in tropics, there are strong sinks in Tropical South America and Tropical Asia, but a very weak sink in Africa. This pattern is significantly different from the estimates of CT2017, but the estimated carbon sinks in each continent and some key regions like Boreal Asia and Amazon are comparable or in the range of previous bottom-up estimates. The inversion also changes the interannual variations of carbon fluxes in most TRANSCOM land regions, which have a better relationship with the changes of severe drought area or LAI, or are more consistent with previous estimates for the impact of drought. These results suggest that the GCASv2 system works well with the GOSAT XCO2 retrievals, and has a good performance in estimating the surface carbon fluxes, meanwhile, our results also indicate that the GOSAT XCO2 retrievals could help to better understand the interannual variations of regional carbon fluxes.