Propagation of planetary-scale disturbances from the lower into the upper atmosphere

Abstract

The possibility that a significant part of the energy of the planetary-wave disturbances of the troposphere may propagate into the upper atmosphere is investigated. The propagation is analogous to the transmission of electromagnetic radiation in heterogeneous media. It is found that the effective index of refraction for the planetary waves depends primarily on the distribution of the mean zonal wind with height. Energy is trapped (reflected) in regions where the zonal winds are easterly or are large and westerly. As a consequence, the summer circumpolar anticyclone and the winter circumpolar cyclone in the upper stratosphere and mesosphere are little influenced by lower atmosphere motions. Energy may escape into the mesosphere near the equinoxes, when the upper-atmosphere zonal flow reverses. At these times tunneling of the energy through a reflecting barrier is also possible. Most of the time, however, there appears to be little mechanical coupling on a planetary scale between the upper and lower atmospheres. Tropospheric sources of wave disturbances in the zonal flow are baroclinic instability and the forcing action of zonally asymmetric heating and topography. The transmissivity of the upper atmosphere increases with wavelength and is greater for the forced perturbations than for the unstable tropospheric waves, whose lengths must be smaller than the critical length for instability. The analysis indicates that baroclinically unstable wave disturbances originating in the troposphere probably do not propagate energy vertically at all. When energy is propagated to great heights, nonlinear vertical eddy transports of heat and momentum associated with the vertically propagating waves should modify the basic zonal flow. However, when the wave disturbance is a small stationary perturbation on a zonal flow that varies vertically but not horizontally, the second-order effect of the eddies on the zonal flow is zero.