Top-down constraint of net carbon exchange in tropical South America

The contribution of vegetation to the South American carbon balance is critical for understanding the regional dynamics in net carbon exchange. Of particular interest is the role of the Amazon region as a sink or source of carbon to the atmosphere. Recent evidence indicate a weakening of the Amazon carbon sink, and when taking fires into account, the region represents a source of carbon to the atmosphere. In this study we use a regional atmospheric inversion system together with data from the Amazon Tall Tower Observatory (ATTO) and airborne profiles of CO₂, to constrain the Net Biome Exchange (NBE) in tropical South America. At the domain-wide scale we find that the atmospheric observations can constrain 64% of the land mass, with uncertainty reductions in most of the Amazon region, and the adjacent Cerrado and Caatinga biomes. Furthermore, we provide a sub-regional-specific analysis showing the effect of assimilating the Amazon Tall Tower Observatory CO₂ time series on the mean seasonal cycle of NBE for four areas within the Amazon, the Cerrado and the Caatinga. An emerging sink-source gradient between the Amazon region (sink) and the integrated effect of the Cerrado and Caatinga (source) is found, but the source is located in the boundaries and outside the eastern border of the legal Amazon. Optimized NBE estimates at regional and subregional scales are shown and the importance of the continuous measurements at ATTO is highlighted. Finally, we indicate the areas with a limited data constraint in our system and conclude that the observational network has to be further expanded for reducing the remaining uncertainty in top-down inverse approaches for this region.