Influences of meridional circulation on driving the SAO in a T213L256 AGCM

Temperature and zonal wind structures around the tropical and subtropical stratopause region have been studied using the Center for Climate System Research/National Institute for Environmental Studies/Frontier Research Center for Global Change (CCSR/NIES/FRCGC) AGCM. The AGCM has T213 spectral truncation in the horizontal and 256 layers in the vertical from the surface up to 85 km (i.e., vertical spacing of 300 m). Gravity waves are spontaneously generated by convection, topography, instability, and so on in the AGCM with no gravity wave drag parameterizations. The AGCM successfully reproduces a quasi-biennial oscillation (QBO)-like oscillation with a period of about 1.5 yr in the tropical stratosphere. The semi-annual oscillation (SAO) reproduced in the AGCM has easterly maxima during the solstices in the lower mesosphere of the summer hemisphere subtropics. An interesting feature is that temperature maxima appear at the subtropical stratopause in the winter hemisphere, simultaneously with the easterly maxima of the stratospheric SAO in the summer hemisphere. In fact, a careful looking at the CIRA-86 climatology shows that the temperature maxima in the winter stratopause are clearly evident. The transformed Eulerian-mean (TEM) analysis has shown that the temperature maxima are brought about by a strong residual-mean meridional circulation composed of a single cell with an ascent in the summer hemisphere tropics (i.e., 0-10 degrees latitude) and a descent in the winter hemisphere extratropics (i.e., around 30 degrees latitude). This meridional circulation is induced by a modest Eliassen-Palm (EP) flux convergence in the winter hemisphere subtropics of the lower mesosphere, which is mainly due to dissipation of extratropical planetary waves. The region of the EP flux convergence has a small absolute vorticity, because of the cancellation between the Coriolis parameter and anticyclonic shear of the zonal wind. Hence, a strong poleward flow is induced in order to cancel a westward acceleration force due to the modest EP flux convergence. The meridional transport of absolute angular momentum associated with the meridional circulation plays a significant role on the seasonal evolution and the formation of easterly maxima of the SAO. On the other hand, upward-propagating gravity and fast Kelvin waves in the tropics can have considerable effects on the downward propagation of easterly and westerly shear of the SAO. The high vertical resolution of the AGCM allows us to quantitatively evaluate effects of gravity and fast Kelvin waves excited by tropical convective systems on the SAO. Relative contributions of the meridional circulation and equatorial gravity and fast Kelvin waves to driving the SAO will be discussed.