Global effects of gravity waves in the middle atmosphere: a theoretical perspective

Abstract The gravity waves observed in the mesosphere must include, and might well be dominated by, waves propagating up from tropospheric or lower-stratospheric sources below. It is such gravity waves — those incident from below — that are responsible for most if not all of the “gyroscopic pumping” that drives the global-scale mesospheric (Murgatroyd—Singleton) mean circulation. Together with the stratospheric (Brewer—Dobson) circulation the mesospheric circulation is crucially important, in turn, to the way in which greenhouse gases are moved around and chemically transformed in the middle atmosphere. The circulation is likely to be important, too, because of possible ways that solar-cycle effects might penetrate downwards from mesospheric altitudes, as well as for the possibility that the summer mesosphere might act, as Thomas (1996) has suggested, as a “miner's canary of global change”. Our understanding both of wave sources and of wave dissipation is rudimentary. For instance it is proving extraordinarily difficult to decide whether or not spontaneous-adjustment emission of gravity waves from tropopause jet streaks (misnamed “geostrophic” adjustment) is a significant wave source or not. The same goes for Kelvin—Helmholtz envelope emission (e.g. Buhler, 1999, Scinocca and Ford, 2000). It is possible, too, that certain kinds of interaction, particularly those between gravity waves and any horizontal inhomogeneity in the background, including tides, are of a non-classical, nondissipative kind only just beginning to be understood theoretically. The current status of our theoretical understanding of these questions will be discussed, with an eye to the space-based observations that are likely to be most important for constraining theoretical thinking and modelling.