On what scales can GOSAT flux inversions constrain anomalies in terrestrial ecosystems?

We investigate the spatial scales over which interannual variability in net ecosystem exchange (NEE) can be constrained using atmospheric CO2 observations from the Greenhouse Gases Observing Satellite (GOSAT). NEE anomalies are calculated by performing a series of inversion analyses using the GEOS-Chem model to assimilate GOSAT observations. Monthly NEE anomalies are compared to "proxies", variables which are associated with anomalies in the terrestrial carbon cycle, and to upscaled NEE estimates from FLUXCOM. Strong agreement is found in the timing of anomalies in the GOSAT flux inversions with soil temperature and FLUXCOM. Strong correlations are obtained in the tropics on continental and larger scales, and in the northern extratropics on sub-continental scales during the summer. These results, in addition to a series of observing system simulation experiments (OSSEs) that were conducted, provide evidence that GOSAT flux inversions can isolate anomalies in NEE on continental and larger scales. However, in both the tropics and northern extratropics, the agreement between the inversions and the proxies/FLUXCOM is sensitive to the flux inversion configuration. In particular, when prior interannual variations in NEE are included in prior fluxes, there is significant degradation in the agreement between the inversions and the proxies/FLUXCOM on sub-continental scales in the northern extratropics. Our results suggest that regional scales are likely the minimum scales that can be resolved in the tropics using GOSAT observations, but even on these scales obtaining robust NEE anomaly estimates may be difficult.