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. Satellite retrievals of the column-averaged dry airmole fractions of CO2 (XCO2) could help to improve carbon flux estimationdue to their good spatial coverage. In this study, in order to assimilate theGOSAT (Greenhouse Gases ObservingSatellite) XCO2 retrievals, the Global Carbon Assimilation System (GCAS) isupgraded with new assimilation algorithms, procedures, a localizationscheme, and a higher assimilation parameter resolution. This upgraded system isreferred to as GCASv2. Based on this new system, the global terrestrial ecosystem(BIO) and ocean (OCN) carbon fluxes from 1 May 2009 to 31 December 2015 areconstrained using the GOSAT ACOS (Atmospheric CO2 Observations from Space) XCO2 retrievals (Version 7.3). Theposterior carbon fluxes from 2010 to 2015 are independently evaluated usingCO2 observations from 52 surface flask sites. The results show that theposterior carbon fluxes could significantly improve the modeling ofatmospheric CO2 concentrations, with global mean bias decreases from aprior value of 1.6 ± 1.8 ppm to −0.5 ± 1.8 ppm. The uncertaintyreduction (UR) of the global BIO flux is 17 %, and the highest monthlyregional UR could reach 51 %. Globally, the mean annual BIO and OCN carbonsinks and their interannual variations inferred in this study are very closeto the estimates of CarbonTracker 2017 (CT2017) during the study period, and the inferred meanatmospheric CO2 growth rate and its interannual changes are also veryclose to the observations. Regionally, over the northern lands, thestrongest carbon sinks are seen in temperate North America, followed by Europe,boreal Asia, and temperate Asia; in the tropics, there are strong sinks intropical South America and tropical Asia, but a very weak sink in Africa.This pattern is significantly different from the estimates of CT2017, butthe estimated carbon sinks for each continent and some key regions likeboreal Asia and the Amazon are comparable or within the range of previous bottom-upestimates. The inversion also changes the interannual variations in carbonfluxes in most TransCom land regions, which have a better relationship withthe changes in severe drought area (SDA) or leaf area index (LAI), or are more consistent withprevious estimates for the impact of drought. These results suggest that theGCASv2 system works well with the GOSAT XCO2 retrievals and shows goodperformance with respect to estimating the surface carbon fluxes; meanwhile, our resultsalso indicate that the GOSAT XCO2 retrievals could help to betterunderstand the interannual variations in regional carbon fluxes.

atmospheric chemistry and physics

Regional CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes from 2010 to 2015 inferred from GOSAT XCO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; retrievals using a new version of the Global Carbon Assimilation System