Regional CO<sub>2</sub> fluxes from 2010 to 2015 inferred from GOSAT XCO<sub>2</sub> retrievals using a new version of the Global Carbon Assimilation System
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abstract
Abstract. Satellite retrievals of the column-averaged dry air
mole fractions of CO2 (XCO2) could help to improve carbon flux estimation
due to their good spatial coverage. In this study, in order to assimilate the
GOSAT (Greenhouse Gases Observing
Satellite) XCO2 retrievals, the Global Carbon Assimilation System (GCAS) is
upgraded with new assimilation algorithms, procedures, a localization
scheme, and a higher assimilation parameter resolution. This upgraded system is
referred 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 are
constrained using the GOSAT ACOS (Atmospheric CO2 Observations from Space) 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. The uncertainty
reduction (UR) of the global BIO flux is 17 %, and the highest monthly
regional UR could reach 51 %. Globally, the mean annual BIO and OCN carbon
sinks and their interannual variations inferred in this study are very close
to the estimates of CarbonTracker 2017 (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, the
strongest carbon sinks are seen in temperate North America, followed by Europe,
boreal Asia, and temperate Asia; in the 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 for each continent and some key regions like
boreal Asia and the Amazon are comparable or within the range of previous bottom-up
estimates. The inversion also changes the interannual variations in carbon
fluxes in most TransCom land regions, which have a better relationship with
the changes in severe drought area (SDA) or leaf area index (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 shows good
performance with respect to estimating the surface carbon fluxes; meanwhile, our results
also indicate that the GOSAT XCO2 retrievals could help to better
understand the interannual variations in regional carbon fluxes.
