Atmospheric CO₂ simulated by the National Center for Atmospheric Research Community Climate Model 1. Mean fields and seasonal cycles

Atmospheric CO₂ has been simulated on-line in a coupled model of the atmosphere and terrestrial land surface. The atmospheric model is a precursor to version 3 of the Community Climate Model developed at the National Center for Atmospheric Research. It interacts with a mechanistic parameterization of the land surface at each time step. Photosynthesis and respiration fluxes are calculated at each time step and are linked to the tracer transport scheme that advects CO₂ through the model atmosphere. For the first time in atmospheric CO₂ modeling, these two fluxes are not forced to balance annually at each grid point. The model was run for 10 years with climatological sea surface temperatures, and the results were saved as monthly means. In this paper, an average year is calculated from the output of the 10 year run and then evaluated in terms of mean distributions and seasonal cycles. The model reproduces many of the observed features of the atmospheric CO₂ distribution although there are discrepancies in some areas. The seasonality of tropical carbon exchange is exaggerated which leads to unrealistic CO₂ near the equator. Carbon uptake is too strong at high northern latitudes in the boreal summer which results in a spurious annual carbon sink and exaggerated seasonal CO₂ cycle amplitudes at these latitudes. The annual sink strongly affects the simulated interhemispheric CO₂ gradient. Errors in the temperature and precipitation simulation cause anomalies in the CO₂ field in specific regions such as the United States and India. Possible model improvements are discussed.