The Influence of Planetary-Wave Transience on Horizontal Air Motions in the Stratosphere

Abstract

The influence of large-scale transience on horizontal air motions and tracer distributions in the stratosphere is explored in equivalent barotropic calculations. Planetary waves are excited by steady and unsteady components of mechanical forcing that are assigned variances typical of variability in the stratosphere. Two classes of transience are considered. A monochromatic traveling wave, representative of discrete components such as the 5- and 16-day waves, is imposed as unsteady mechanical forcing. The system is also forced by a second-order stochastic process representative of broadband variability. The response to each of these forms of unsteady forcing is investigated in terms of the characteristic time scale of the transience. For monochromatic transience, eddy transport is concentrated inside the critical region of the traveling wave, for example, where perturbation velocities become comparable to the Doppler-shifted flow. There, eddy displacements are large enough for nonconservative behavior and net transport to occur, which leave a permanent influence on the circulation and tracer distributions. Eddy transport is greatest for low-frequency disturbances because the critical region of the traveling wave then overlaps that of the stationary wave. During constructive interference, eddy displacements of the traveling wave reinforce those of the stationary wave. The nonlinear interaction that takes place between the two components leads to an expanded critical region and more extensive transport. In contrast, high-frequency traveling waves have large Doppler-shifted flows everywhere. Air motions associated with such disturbances are nearly reversible, leaving only a small permanent influence on the circulation and tracer distributions. For stochastic transience, all frequencies are present. The critical region and eddy transport are then smeared across the globe. As a result, eddy transport is weaker locally than that concentrated inside the critical region of a monochromatic traveling wave with the same variance. Only those spectral components slow enough to be Doppler shifted to small intrinsic phase speeds exert a lasting influence on the circulation. Consequently, transience distributed over a wide range of frequency (e.g., spectrally ?white?) produces less overall transport than transience concentrated at low frequencies (e.g., spectrally ?red?). Stochastic forcing also excites westward-propagating transients that radiate into the summer hemisphere and disperse globally into planetary normal modes. Favored in the response to broadband forcing, those discrete components lead to behavior similar to that of traveling waves excited by monochromatic forcing. By introducing nonconservative behavior in regions where they reinforce large displacements of the stationary wave and where they themselves are Doppler shifted to small intrinsic phase speeds, these unsteady components can contribute to the momentum budgets of both the wintertime and the summertime circulations.