We use the mean meridional tropical circulation of the Atmospheric Ocean Coupled General Circulation Models (AOGCM) to diagnose and quantify the modifications of the mean meridional circulation of the atmosphere under global warming conditions. The AOGCMs generally show a weakening of the Hadley circulation for the winter cell in both hemispheres, accompanied by a poleward extension of the Hadley circulation area. The conditions explaining these modifications are analysed using detailed outputs from IPSL-CM4. The AOGCM IPSL-CM4 shows changes, under CO2 doubling, that are in accordance with the other models, for austral winter. On the other hand, for boreal winter, the winter cell shows little change in intensity and in extension. The poleward shift of the Southern Hemisphere winter Hadley cell corresponds to changes in the transient eddies, whereas the increase of the dry static stability is mainly responsible for the mean weakening of the cell. For boreal winter, a strong shrinking of the ascending branch area, and an increase of the latent heating, is found to cancel the weakening of the circulation due to dry static stability increase.