Comparing global models of terrestrial net primary productivity (NPP): analysis of the seasonal atmospheric CO2 signal

Eight terrestrial biospheric models (TBMs) calculating the monthly distribu tions of both net primary productivity (NPP) and soil heterotrophic respira tion (R-H) in the Potsdam NPP Model Intercomparison workshop are used to si mulate seasonal patterns of atmospheric CO2 concentration. For each model, we used net ecosystem productivity (NEP=NPP-R-H) as the source function in the TM2 atmospheric transport model from the Max-Planck Institute for Meteo rology. Comparing the simulated concentration fields with detrended measure ments from 25 monitoring stations spread over the world, we found that the decreasing seasonal amplitude from north to south is rather well reproduced by all the models, though the amplitudes are slightly too low in the north . The agreement between the simulated and observed seasonality is good in t he northern hemisphere, but poor in the southern hemisphere, even when the ocean is accounted for. Based on a Fourier analysis of the calculated zonal atmospheric signals, tropical NEP plays a key role in the seasonal cycle o f the atmospheric CO2 in the whole southern hemisphere. The relatively poor match between measured and predicted atmospheric CO2 in this hemisphere su ggests problems with all the models. The simulation of water relations, a d ominant regulator of NEP in the tropics, is a leading candidate for the sou rce of these problems.