Mean meridional circulations in the middle atmosphere

grantor: University of TorontoIn this thesis are presented results describing features of zonal-mean dynamics in the middle atmosphere which lie outside the framework described by the "downward control" principle. This principle applies to the nearly steady state wave-driven regime that is approached in the extratropics during solstice seasons. However, it does not apply in the tropics and transience in the extratropics can be significant. One of the assumptions underlying the downward control principle is that local changes in wave drag or diabatic heating do not significantly modify wave drag elsewhere. Force feedbacks are investigated in the particular case of artificial "sponge layers" used in general circulation models. Such relaxational damping is found to couple with changes in forcing or diabatic heating in an unrealistic manner, leading to "upward control". In the tropics there exists the so-called nonlinear Hadley circulation, which is driven by a combination of gradient-wind balance and solar heating gradients rather than wave forcing. Previous work is extended by numerical experiments which support the validity of gradient-wind balance in the tropics. Inertial instability is found to have a potentially large impact on the structure of this circulation. Another purpose of this work is to explain annual mean upwelling in the tropical stratosphere, an important element in the middle atmosphere climate. The inviscid linear view of the diabatic meridional circulation induced by extratropical wave forcing cannot explain the observed mean upwelling. The nonlinear Hadley circulation is found to make a large contribution to upwelling in the tropical upper stratosphere. In the tropical lower stratosphere, upwelling is produced by the wave-driven circulation. A nonlinear mechanism for this is demonstrated by numerical experiments. The dynamical impact of spatio-temporal variation in the radiative damping rate is also investigated. In regions where the damping rate is weak there is increased lag between the evolution of wave forcing and the induced circulation. In such regions the temperature is also more sensitive to even a weak circulation. A time dependence in the radiative damping rate is shown to produce a transient-like signature in the annually averaged dynamical fields, which complicates the interpretation of annual mean anomalies.Ph.D