SIMULATIONS OF TERRESTRIAL CARBON METABOLISM AND ATMOSPHERIC CO2 IN AGENERAL-CIRCULATION MODEL .1. SURFACE CARBON FLUXES

The exchange of carbon dioxide (CO2) between the atmosphere and terres trial ecosystems due to photosynthesis and respiration has been simula ted using a new version of the simple biosphere model (SiB2) and the C olorado State University (CSU) general circulation model (GCM). Parame ters associated with the extent and seasonality of vegetation were der ived from satellite observations. The fluxes were calculated at the GC M time step of 6 min, so that the diurnal cycle of photosynthesis is w ell resolved. Annual net primary productivity simulated by the coupled model agrees well with previous estimates in most regions of the worl d. In some regions (central North America, southeastern South America, southeast Asia), the precipitation simulated by the CSU GCM is less t han observed, and in those regions the simulated NPP is less than prev ious estimates. The amplitude of the seasonal cycle of the simulated n et flux is quite similar to previous estimates, but the phase is signi ficantly earlier in the northern temperate and boreal zones, both as s imulated by the GCM and when SiB2 is driven off-line using observed me teorological forcing. At the few locations for which observational dat a are available, the phase of the simulated seasonal cycle of net carb on fluxes agrees well with the data, but at one temperate forest grid cell the amplitude is too low. The phase of the simulated diurnal cycl e 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 str ess due to the less-than-observed simulated precipitation.