A comprehensive global three-dimensional model of δ18O in atmospheric CO2: 2. Mapping the atmospheric signal

We have modeled the distribution of δ18O in atmospheric CO2 with a new comprehensive global three‐dimensional model. We have focused in this study on the seasonal cycle and the meridional gradient in the atmosphere. The model has been compared with a data set of δ18O‐CO2, which merges measurements m...

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Bibliographic Details
Published in:Journal of Geophysical Research - Atmospheres 2003-09, Vol.108 (D17), p.4528-n/a
Main Authors: Cuntz, Matthias, Ciais, Philippe, Hoffmann, Georg, Allison, Colin E., Francey, Roger J., Knorr, Wolfgang, Tans, Pieter P., White, James W. C., Levin, Ingeborg
Format: Article
Language:English
Subjects:
18O in CO2
Atmosphere
Atmospheric Composition and Structure
Biogeochemical processes
Biosphere/atmosphere interactions
Constituent sources and sinks
Global Change
global model
isotope feedback mechanism
isotope model
Mathematical Geophysics
Modeling
rectifier effect
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Summary:We have modeled the distribution of δ18O in atmospheric CO2 with a new comprehensive global three‐dimensional model. We have focused in this study on the seasonal cycle and the meridional gradient in the atmosphere. The model has been compared with a data set of δ18O‐CO2, which merges measurements made by different laboratories, with allowance for recently elucidated calibration biases. The model compares well with the seasonal cycle of CO2, but advances the measured δ18O‐CO2 seasonal cycle by two months. The calculated seasonal amplitude is typically 2/3 of the measured value, but the sensitivity to uncertainties in the input parameter set is such that a range of amplitudes over a factor of 3 is accommodated. Unlike the case for the amplitude, the sensitivity analyses demonstrate that the modeled phase of the seasonal cycle and the north‐south gradient are practically unaffected by uncertainty in the parameter set. The north‐south gradient comes, on the one hand, from the disequilibrium of the δ18O‐CO2 isofluxes at every grid point and, on the other hand, from rectification gradients, a covariance of the varying δ18O‐CO2 source with the atmospheric transport. The model exhibits a very strong rectification gradient that can lead to a misinterpretation of the measurements compared to the model. We therefore restrict comparison to the latitudinal means of only ocean grid cells with measurements from stations sampling the marine boundary layer. Assimilation and respiration are the determining factors of the seasonal cycle and the north‐south gradient of δ18O‐CO2. In a number of sensitivity studies we have explored the range of possible processes affecting the simulated seasonal cycle and hemispheric gradient. None of these processes contributed significantly to improve the model‐observation mismatch. The contribution of assimilation and respiration to the total signal does change significantly in the sensitivity studies, but, because of feedback processes, they change in such a way that the overall response of the model is only marginally altered. In particular, prescribing δ18O‐H2O soil values to monthly means of rain does not significantly change the modeled signal, either in the seasonal cycle or in the meridional gradient. This highlights the need to accurately model assimilation and respiration in order to understand δ18O in atmospheric CO2.
ISSN:0148-0227
2156-2202
DOI:10.1029/2002JD003154