The atmospheric CO2 seasonal cycle in the Northern Hemisphere is caused mainly by the seasonal growth and decay of the vegetation. Previous studies have attributed an increase in the seasonal cycle amplitude to an increase in the biospheric assimilation of atmospheric CO2 stimulated, particularly in high latitudes, by a warming climate and/or by land disturbances. Here we offer another plausible mechanism. Using a 3-dimensional atmospheric transport model driven by ECMWF winds, we show that shifts in the atmospheric circulation regime can also cause inter-annual variations and quasi-decadal trends in the CO2 seasonal cycle amplitude that are of similar magnitude as those observed at Mauna Loa, Pt. Barrow and Alert from 1979 to 1998, independent of changes in the CO2 surface fluxes. Our results demonstrate the importance of the need to use observed winds when carrying out interpretations of observed inter-annual atmospheric CO2 variability in terms of surface flux changes.