Contribution of ocean, fossil fuel, land biosphere, and biomass burning carbon fluxes to seasonal and interannual variability in atmospheric CO₂

Seasonal and interannual variability in atmospheric carbon dioxide (CO₂) concentrations was simulated using fluxes from fossil fuel, ocean and terrestrial biogeochemical models, and a tracer transport model with time-varying winds. The atmospheric CO₂ variability resulting from these surface fluxes was compared to observations from 89 GLOBALVIEW monitoring stations. At northern hemisphere stations, the model simulations captured most of the observed seasonal cycle in atmospheric CO₂, with the land tracer accounting for the majority of the signal. The ocean tracer was 3-6 months out of phase with the observed cycle at these stations and had a seasonal amplitude only ~10% on average of observed. Model and observed interannual CO₂ growth anomalies were only moderately well correlated in the northern hemisphere (R ~ 0.4-0.8), and more poorly correlated in the southern hemisphere (R < 0.6). Land dominated the interannual variability (IAV) in the northern hemisphere, and biomass burning in particular accounted for much of the strong positive CO₂ growth anomaly observed during the 1997-1998 El Niño event. The signals in atmospheric CO₂ from the terrestrial biosphere extended throughout the southern hemisphere, but oceanic fluxes also exerted a strong influence there, accounting for roughly half of the IAV at many extratropical stations. However, the modeled ocean tracer was generally uncorrelated with observations in either hemisphere from 1979-2004, except during the weak El Niño/post-Pinatubo period of the early 1990s. During that time, model results suggested that the ocean may have accounted for 20-25% of the observed slowdown in the atmospheric CO₂ growth rate.