Abstract. The Orbiting Carbon Observatory-2 has been on orbit since 2014, and its global coverage holds the potential to reveal new information about the carbon cycle through the use of top-down atmospheric inversion methods combined with column average CO2 retrievals. We employ a large ensemble of atmospheric inversions utilizing different transport models, data assimilation techniques, and prior flux distributions in order to quantify the satellite-informed fluxes from OCO-2 Version 7r land observations and their uncertainties at continental scales. Additionally, we use in situ measurements to provide a baseline against which to compare the satellite-constrained results. We find that within the ensemble spread, in situ observations, and satellite retrievals constrain a similar global total carbon sink of 3.7±0.5 PgC yr−1, and 1.5±0.6 PgC yr−1 for global land, for the 2015–2016 annual mean. This agreement breaks down in smaller regions, and we discuss the differences between the experiments. Of particular interest is the difference between the different assimilation constraints in the tropics, with the largest differences occurring in tropical Africa, which could be an indication of the global perturbation from the 2015–2016 El Niño. Evaluation of posterior concentrations using TCCON and aircraft observations gives some limited insight into the quality of the different assimilation constraints, but the lack of such data in the tropics inhibits our ability to make strong conclusions there.

Movie S1Movie S1. Results of the assimilation of OCO-2 XCO2 data into a high-resolution global model for March 2015 through July 2015, highlighting the springtime reduction in atmospheric CO2.

Spaceborne observations of carbon dioxide (CO) from the Orbiting Carbon Observatory-2 are used to characterize the response of tropical atmospheric CO concentrations to the strong El Niño event of 2015-2016. Although correlations between the growth rate of atmospheric CO concentrations and the El Niño-Southern Oscillation are well known, the magnitude of the correlation and the timing of the responses of oceanic and terrestrial carbon cycle remain poorly constrained in space and time. We used space-based CO observations to confirm that the tropical Pacific Ocean does play an early and important role in modulating the changes in atmospheric CO concentrations during El Niño events-a phenomenon inferred but not previously observed because of insufficient high-density, broad-scale CO observations over the tropics.