INFLUENCE OF 2 ATMOSPHERIC TRANSPORT MODELS ON INFERRING SOURCES AND SINKS OF ATMOSPHERIC CO2

Atmospheric transport models are a source of uncertainty in the diagno stics of the CO2 sources and sinks. We propose here a protocol to comp are two transport models: a 2-dimensional (2D) and a 3-dimensional (3D ) model based on 3 different experiments that reveal the ability of ea ch model to account for the different components of the atmospheric ca rbon cycle. The 2D model we use is the one described by Tans et al. an d the 3D model is the TM2 model, developed by Heimann et al. First, we conduct the same fossil fuel experiment in both models and show that the 2D model has a stronger inter-hemispheric mixing than the 3D model (similar to 25%), even though the 2D model presents a weaker intra-he mispheric mixing above source regions (experiment A). The influence of year-to-year variability of transport on the latitudinal profile in f ossil-fuel CO2 appears to be weak for the 1990s. We then use a set of ''all but fossil fuel'' fluxes, originally inferred from the 2D model, as an input to the 3D model (experiment B). Even if the main discrepa ncy on the resulting latitudinal CO2 concentrations occurs between the 2D and 3D models in the tropics and at the mid-northern latitudes, th e differences implied by three longitudinal distributions tested in th e 3D model are important and can be explained by a few global transpor t mechanisms. Finally, we quantify the differences in latitudinal CO2 concentrations observed in experiment B in terms of net carbon fluxes at the surface. To do so, an inverse calculation of the CO2 fluxes in latitude and time is performed with the 3D model, using as an input a smoothed latitudinal profile of atmospheric measurements for the perio d 1990-1993 (experiment C=A+B). We find with the 3D model that, averag ed on the period 1990-93, the equatorial release is reduced by 40 Tmol yr(-1) (roughly 25% of the original source) compared with the initial 2D budget and is shifted southward by roughly 10 degrees. The mid nor thern latitude sink is also reduced by 80 Tmol yr(-1) (roughly 25% of the original sink). In summary, this study shows that the changes in t he carbon budget required when moving from the 2D model to this 3D mod el are important, but they are not radical changes.