Abstract
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.
Highlights
The large-scale atmospheric circulation in the tropics constitutes a key element of the tropical climate
We have focused our diagnostics on two simulations that are part of Coupled Model Intercomparaison Project Phase 3 (CMIP3): (1) a control simulation, referred to as 1CO2, where all the greenhouse gases are set to their pre-industrial concentrations and (2) a climate change simulation in response to an idealized 1% yearly increase of the carbon dioxide (CO2) concentration followed by a 40-yr stabilization at a doubled CO2 concentration
The parametrized physics is well validated on tropical climate, and the use of IPSL-CM4 is appropriate for tropical climate studies
Summary
The large-scale atmospheric circulation in the tropics constitutes a key element of the tropical climate. The tropical meridional circulation, that is, the Hadley circulation is expected to decrease under global warming Mitas and Clement (2006) examined the Hadley circulation of the 20th century AR4 simulations, and showed a modest decrease of the Northern Hemisphere wintertime (DJF) streamfunction They explained it by the increase of the static stability in the GCMs. Lu et al (2007) diagnosed a weakening of the Hadley circulation in the scenario simulation of global warming, which is in the order of ∼1% per K of surface warming.
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