Seasonal and inter-annual variability of carbon fluxes in southern Africa seen by GOSAT

Abstract. The inter-annual variability of the global carbon sink is heavily influenced by semi-arid regions. Southern hemispheric Africa has large semi-arid and arid regions. However, there is only a sparse coverage of in situ CO₂ measurements on the southern hemisphere. This leads to uncertainties in measurement-based carbon flux estimates for these regions. Also, dynamic global vegetation models (DGVMs) show large inconsistencies in semi-arid regions. Satellite CO₂ measurements offer a spatially extensive and independent source of information about the southern African carbon cycle.

We examine Greenhouse Gases Observing Satellite (GOSAT) CO₂ concentration measurements from 2009 to 2018 in southern Africa. We infer CO₂ land-atmosphere fluxes which are consistent with the GOSAT measurements using the atmospheric inversion system TM5-4DVar. We find systematic differences between these satellite-based carbon fluxes and atmospheric inversions based on in situ measurements pointing towards a limited measurement information content in the latter. We use the GOSAT based fluxes and additionally Solar Induced Fluorescence (SIF), a proxy for photosynthesis, as atmospheric constraints to select DGVMs of the TRENDYv9 ensemble which show compatible fluxes. The selected DGVMs allow for studying the vegetation processes driving the southern African carbon cycle. We show that the variability of photosynthetic uptake mainly drives the inter-annual variability of the southern African carbon fluxes. The seasonal cycle, however, is substantially influenced by enhanced soil respiration due to soil rewetting at the beginning of the rainy season. The latter result emphasizes the importance of correctly representing the response of semi-arid ecosystems to soil rewetting in DGVMs.