SIMULATIONS OF TERRESTRIAL CARBON METABOLISM AND ATMOSPHERIC CO2 IN AGENERAL-CIRCULATION MODEL .2. SIMULATED CO2 CONCENTRATIONS

The effects of terrestrial photosynthesis and respiration on the mixin g ratio of atmospheric CO2 have been simulated using surface fluxes ca lculated using a new version of the simple biosphere model (SiB2) coup led to the Colorado State University (CSU) general circulation model ( GCM). The model was integrated for 5 years from an initial condition o f uniform CO2, with surface fluxes and atmospheric transport calculate d on a 6-min time step, Subgrid-scale vertical transport includes the effects of cumulus and dry convection and boundary layer turbulence, w ith diurnal cycles of all processes well resolved. The amplitude and p hase of the diurnal cycle of simulated CO2 concentration during the gr owing season agreed very well with observations made in Brazil, the so utheastern United States, and central Canada, and the vertical structu re of the simulated diurnal variations of CO2 in the lower troposphere appears to be fairly realistic. By contrast, when the model was drive n with surface fluxes of CO2 derived from monthly means saved from the on-line simulation, the diurnal cycle was much weaker than observed a t all three locations and was nearly 180 degrees out of phase with the observations. The amplitude and phase of the seasonal cycle of simula ted concentration show good agreement with data collected in remote ma rine areas by the flask sampling network. Vertical attenuation of the seasonal amplitude in the model is stronger than observed, at least ov er the western Pacific ocean where seasonal data have been collected b y aircraft. In the annual mean, correlations between the carbon fluxes and vertical atmospheric transport produce very strong concentration maxima over the tropical rain forests, but covariance of fluxes and tr ansport on the seasonal time scale are more important in the middle la titudes. The effect of these correlations is to impose a vertical grad ient of several parts per million on the zonal mean atmospheric CO2 co ncentration over biologically active regions, with the seasonal cycle contributing about 75% of the effect and the diurnal cycle contributin g about 25%. The simulated annual mean meridional gradient in concentr ation at the flask stations is much stronger than has been simulated w ith off-line tracer transport models, accounting for more than half of the observed north-south gradient.