MEAN MERIDIONAL CIRCULATION DRIVEN BY EDDY FORCINGS OF DIFFERENT TIMESCALES

In this paper, the mean meridional circulation (MMC) forced by eddy fl uxes of heat and momentum is examined using a simple quasigeostrophic, two-level model of the zonal-mean atmosphere. Analytic solutions have been obtained, which show that analyses of the eddy-induced MMC using the Kuo-Eliassen equation are most appropriate for high-frequency flu ctuations. For steady-state or low-frequency fluctuations, the eddy fl uxes will lead to changes in the zonal-mean zonal wind and temperature . These changes in the zonal-mean state will induce changes in frictio nal dissipation and diabatic heating, which (together with the eddy fl uxes) are constrained to satisfy a generalization of the Eliassen-Palm theorem and will also act as source terms to the Kuo-Eliassen equatio n. The inclusion of these induced terms usually leads to a significant enhancement in the diagnosed intensity of the MMC. This can explain w hy previous studies of the MMC found a much weaker eddy-induced Ferrel cell than that observed when the induced frictional and diabatic heat ing terms were left out and the eddy fluxes only were used as source t erms. The relevant timescale separating the high- and low-frequency li mits is found to be the radiative timescale in me model.