The mean-meridional circulation and wave-mean flow interactions in the pressure-isentrope hybrid coordinate (pt) are investigated through an analysis of an annual run of a general circulation model (NCAR CCM1). Special attention is paid to seasonal variations of the circulation and their comparisons between the northern and southern hemispheres. In the troposphere, there are two types of direct circulations, namely, the Hadley cells in low latitudes and direct cells in the extratropics. The latter are almost confined to the troposphere, become strong in winter and have nearly equal magnitude in the two winter hemispheres. These findings indicate that the extratropical cells come mainly from baroclinic instability waves. The stratospheric circulation is of the Brewer-Dobson type and, however, its seasonal evolution is considerably different between the two hemispheres due to the difference in wave activity. The extratropical downward flows in the NH winter are stronger than those in the SH winter and, hence, the global stratosphere-troposphere exchange rate of the air mass has a maximum in the NH winter. Especially inside of the SH circumpolar vortex, downward flows are so weak that the lower stratospheric temperature becomes very low. Such NH-SH asymmetries of downward flows in winter may cause the seasonal variation of the stratospheric temperature even in the tropics through changing tropical upward flows.
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