Abstract

This lecture surveys two key aspects of the dynamics of global middle-atmospheric circulations and stratosphere-troposphere mass exchange rates, with their implications for temperatures (departing systematically from radiative equilibrium) and for the transport, distribution, and photochemical evolution of greenhouse gases. First, there is an overwhelming case that the tropical stratospheric quasi-biennial oscillation (QBO) is a wave-driven mean flow, though much uncertainty as to which waves are most important in driving it. The QBO is the most conspicuous illustration of the way in which wave fields can act ``anti-frictionally'', driving the atmosphere away from solid rotation, not toward it in the manner of a viscosity. Second, the global stratospheric (Brewer-Dobson) and mesospheric (Murgatroyd-Singleton) mean circulations are wave-driven. Nothing else can explain for instance the refrigeration of the summer polar mesopause; and nothing else is fluid-dynamically possible in a strongly stratified, rapidly rotating, radiatively relaxational atmosphere under anything like the observed conditions. This driving can be thought of as a mechanical ``gyroscopic pumping''. Again, there is uncertainty about exactly how the pumping action is shared between Rossby waves and gravity waves, and how this, and anti-frictional effects, might impact the robustness or otherwise of e.g. tropical stratospheric upwelling -- the part of the system that ``tape-records'' the annual tropopause water-vapour cycle. The mesosphere has special importance not only as a sensitive detector of gravity-wave fields but also, potentially, of such things as long-term trends in water vapour and of poorly understood, or unsuspected, mechanisms related to solar variability.

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