. A new decadal atmosphere-based estimate on North American carbon uptake from a high-resolution regional inverse model: CarbonTracker-Lagrange

Abstract
North America (NA) is an important source and terrestrial sink for atmospheric CO2. However, uncertainties on NA terrestrial uptake 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 NA terrestrial CO2 fluxes. Here, we present a decadal estimate of NA terrestrial uptake from a high resolution inversion framework that uses a dense network of highly accurate atmospheric CO2 observations over NA. We utilize observation system simulation experiments and independent flux measurements to demonstrate improved flux estimates using high resolution regional models compared to fluxes derived from coarser resolution global models. Although our derived multi-year annual net terrestrial flux is consistent with that estimated from global models, significant differences were observed in spatiotemporal distribution of fluxes. In particular, a consistent response of NA terrestrial uptake to 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 this consistent response is primarily due to the modulation of hydrological conditions over NA associated with ENSO.