Three‐dimensional tracer model study of atmospheric CO₂: Response to seasonal exchanges with the terrestrial biosphere

A three‐dimensional tracer transport model is used to investigate the annual cycle of atmospheric CO 2 concentration produced by seasonal exchanges with the terrestrial biosphere. The tracer model uses winds generated by a global general circulation model to advect and convect CO 2 ; no explicit diffusion coefficients are employed. A biospheric exchange function constructed from a map of net primary productivity, and Azevedo's (1982) seasonality of CO 2 uptake and release closely simulates the annual cycles at coastal stations. The results show that zonal homogeneity in surface CO 2 concentrations can never be achieved at mid‐latitudes where the time scale for zonal mixing is longer than the time scale for biospheric exchange. Analysis of the zonal mean balance in the lower troposphere reveals that atmospheric transport processes may alter the CO 2 response to local biospheric exchanges by 50% or more. Hence year‐to‐year variation of the annual CO 2 cycle may result from the natural variability of the atmospheric circulation as well as from changes in the sources and sinks.