A decadal atmosphere-based estimate on North American terrestrial uptake

North America is an important source and terrestrial sink for atmospheric CO₂. However, uncertainties on North American terrestrial CO₂ fluxes are large, including their magnitude, distribution, inter-annual variability, and trend. Given such large uncertainties, it has been challenging to identify coherent relationships between climate drivers and North American terrestrial CO₂ fluxes. Here, we will discuss a decadal estimate of North American terrestrial uptake from a high resolution inversion framework, CarbonTracker-Lagrange that uses a dense network of highly accurate atmospheric CO2 observations over North America. We will demonstrate improvement of North American CO₂ flux estimates using a high resolution regional model compared to fluxes derived from a coarser resolution global model using observation system simulation experiment (OSSEs) and independent flux measurements. Although our derived multi-year annual net terrestrial flux from North America is consistent with that estimated from global inverse models for 2007 - 2015, significant differences were observed in spatiotemporal distribution of fluxes. In particular, a consistent response of North American terrestrial uptake to large global climate patterns such as the El Niño and Southern Oscillation (ENSO) emerges from our new flux results. Such persistent responses were shown in atmospheric CO₂ and δ¹³CO₂ observations, as well as in fluxes measured by eddy covariance. By analyzing environmental driver data, we find the response of North American terrestrial uptake to ENSO is primarily due to the modulation of hydrological conditions over North American associated with ENSO.