Interannual variation in the atmospheric CO2 growth rate: Role of atmospheric transport in the Northern Hemisphere

In this study, we examine the role of the atmospheric circulation variability in reproducing the interannual variability of the observed atmospheric CO2 growth rate in the Northern Hemisphere. Results from a series of numerical experiments performed with a three‐dimensional atmospheric transport model driven by European Centre for Medium‐Range Weather Forecasts winds from 1979 to 1999 and forced at the bottom with an annually balanced, seasonally distributed (i.e., no interannual variations) terrestrial CO2 flux calculated by the Carnegie‐Ames‐Stanford Approach (CASA) ecosystem model show changes in atmospheric CO2 growth rates that are significantly correlated with those observed at many stations located in midlatitude to high‐latitude regions in the Northern Hemisphere. Particularly, we find that the North Atlantic Oscillation (NAO) and the Pacific‐North America (PNA) indices are correlated with the observed growth rates at Alert and Point Barrow. This suggests that where there is large variability in atmospheric circulation, it needs to be taken into account when changes in observed atmospheric CO2 at various stations are to be understood in terms of changes in CO2 sources. In numerical experiments in which biospheric CO2 fluxes from the CASA model are perturbed in a known arbitrary way, we find that the Northern Hemisphere observational sites do not appear to be sensitive to changes in the CO2 seasonal flux cycle except at close proximity and thus would not be useful for inferring a Northern Hemisphere biospheric sink. Results suggest sensitivity of the locations of some monitoring stations, with respect to atmospheric circulation patterns, in detecting major changes in the biospheric CO2 flux from certain regions in the Northern Hemisphere.