Simulations of terrestrial carbon metabolism and atmospheric CO2 in a general circulation model .1. Surface carbon fluxes

The exchange of carbon dioxide (CO2) between the atmosphere and terrestrial ecosystems due to photosynthesis and respiration has been simulated using a new version of the simple biosphere model (SiB2) and the Colorado State University (CSU) general circulation model (GCM). Parameters associated with the extent and seasonality of vegetation were derived from satellite observations. The fluxes were calculated at the GCM time step of 6 min, so that the diurnal cycle of photosynthesis is well resolved. Annual net primary productivity simulated by the coupled model agrees well with previous estimates in most regions of the world. In some regions (central North America, southeastern South America, southeast Asia), the precipitation simulated by the CSU GCM is less than observed, and in those regions the simulated NPP is less than previous estimates. The amplitude of the seasonal cycle of the simulated net flux is quite similar to previous estimates, but the phase is significantly earlier in the northern temperate and boreal zones, both as simulated by the GCM and when SiB2 is driven off-line using observed meteorological forcing. At the few locations for which observational data are available, the phase of the simulated seasonal cycle of net carbon fluxes agrees well with the data, but at one temperate forest grid cell the amplitude is too low. The phase of the simulated diurnal cycle reproduces observations from a temperate forest, temperate grassland, and tropical forest. The amplitude of the simulated diurnal cycle is close to the observed amplitude early in the season at the temperate grassland site, but deteriorates by late summer because of drought stress due to the less-than-observed simulated precipitation.