A three dimensional model of atmospheric CO₂transport based on observed winds: 3. Seasonal cycle and synoptic time scale variations

The spatial and temporal distribution of atmospheric CO2 and of its 13C/12C ratio have been simulated with a three-dimensional atmospheric transport model based on the observed wind fields of the Global Weather Experiment in 1978/1979. Modeled sources and sinks of carbon at the surface of the Earth include: (1) CO2 released from fossil fuel combustion, (2) exchange of CO2 with the terrestrial biosphere by specification of net primary productivity from satellite data and heterotrophic respiration from surface temperature, (3) air-sea exchange of CO2 driven by prescribed temporal and spatial variations of CO2 in the surface waters. The relative contributions of individual source components are calculated by running the model with each component separately. This paper describes the results of the model simulations in terms of the seasonal cycle and synoptic time scale variations. Acceptable agreement with observations is achieved for northern hemisphere and tropical stations. In the southern hemisphere the model overestimates the observed seasonal cycle. The relationship between the 13C/12C isotopic ratio and CO2 concentration is discussed as a potential means to distinguish biological and oceanic contributions to the seasonal cycle.