Abstract: Upon investigating the relative locations of internal and external forcing and the resultant mean meridional circulation,it is found that thermal forcing and mechanical forcing for the formation of atmospheric mean meridional circulation are modulated by a certain ratio.This ratio is determined by the inherent baroclinity,static stability and absolute vorticity of the atmosphere.By employing a parameterization scheme for radiative heating and condensation heating,together with the analysis data of the European Center for Medium-range Weather Forecasts,the mean meridional circulation for January was simulated numerically.It was found that latent heat release in the tropics may result in the formation of double-layered Hadley circulation,50 may the eddy momentum transfer processes.On the other hand,mean meridional circulations in extra-tropics are mainly determined by external momentum forcing and atmospheric properties of eddy momentum and heat transfer.