Dj. Erickson et al., THE SEASONAL CYCLE OF ATMOSPHERIC CO2 - A STUDY BASED ON THE NCAR COMMUNITY CLIMATE MODEL (CCM2), JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 101(D10), 1996, pp. 15079-15097
A global three-dimensional atmospheric model, the NCAR CCM2 general ci
rculation model, has been adapted to study the hourly to yearly variab
ility of CO2 in the atmosphere. Features of this CCM2-based model incl
ude high spatial resolution (2.8 degrees x 2.8 degrees latitude/longit
ude), 18 vertical levels, a 15-min time step, and an explicit, nonloca
l atmospheric boundary layer parameterization. The surface source/sink
relationships used include exchange with the ocean, the terrestrial b
iosphere, biomass burning, and fossil fuel release of CO2. The timing
and magnitude of the model seasonal cycle are compared to observationa
l data for 28 sites. The seasonal cycle of atmospheric CO2 is generall
y well predicted by the model for most of the northern hemisphere, but
estimates of the amplitude of the seasonal cycle in the southern hemi
sphere are overpredicted. To address this aspect more rigorously, we h
ave used the monthly surface ocean pCO(2) maps created by the Max-Plan
ck-Hamburg ocean general circulation model to asses the ocean seasonal
ity on the atmospheric surface CO2 seasonality. The globally averaged
interhemispheric gradient in atmospheric CO2 concentrations, as comput
ed with the chosen source/sink distributions, is a factor of two too h
igh compared to data, and selected longitudinal bands may be up to 50%
higher than the zonal mean. The high temporal resolution of this mode
l allows the infrequent yet real extrema in atmospheric CO2 concentrat
ions to be captured. The vertical attenuation of the seasonal cycle of
atmospheric CO2 is well simulated by the boundary layer/free troposph
ere interaction in the model in the northern hemisphere. Conversely, a
n increasing amplitude of the seasonal cycle aloft is found in the mid
latitude southern hemisphere indicating interhemispheric transport eff
ects from north to south. We use two different models of the terrestri
al biosphere to examine the influence on the computed seasonal cycle a
nd find appreciable differences, especially in continental sites. A gl
obal three-dimensional chemical transport model is used to assess the
production of CO2 from the oxidation of CO throughout the volume of th
e atmosphere. We discuss these CO + OH --> CO2 + H results within the
context of inverse model approaches to ascertaining the global and reg
ional source/sink patterns of CO2. Deficiencies in the model output as
compared to observational data are discussed within the context of gu
iding future research.