Downward propagation of dynamical signals in the middle atmosphere

Studies of the effect of changes in the middle atmosphere on the troposphere have suggested that there may be two distinct phases to the response to such changes. One is a downward propagation of the response in the stratosphere, and the second is an interaction between the lower stratosphere and the troposphere. The work reported here concentrates on the first phase.

Experiments are carried out using a Holton-Mass type wave mean flow model to investigate the downward influence within the stratosphere of a dynamical perturbation introduced into the zonal wind field in the upper stratosphere. It is found that no simple downward propagating dynamical signal is supported by the mean flow dynamics but rather the strongly non-linear nature of the dynamics are sensitive to this perturbation and the resulting change occurs over the whole depth of the stratosphere. The dependence of the downward penetration of this response both to the amplitude of wave forcing at the lower boundary and to the time scale of radiative relaxation to an imposed background temperature profile is investigated. The Holton-Mass model supports steady and vacillating equilibria which gain or lose stability depending on the amplitude of the lower boundary wave forcing. We find that the amount of downward influence seen is greatest when multiple stable equilibria exist. We also find that a degree of non-locality in the zonal mean dynamics (naturally present in the extratropics) is required for any downward propagation to occur as is some wave forcing at low level.

Previous work with three-dimensional mechanistic (stratosphere-mesosphere) models has also shown the possibility, for certain ranges of large-scale wave forcing, of multiple steady states and vacillations (e.g. Scott and Haynes 2000, Christiansen 2003) and sensitivity to initial conditions (Gray et al 2003). The study using the Holton-Mass model is therefore being continued to investigate in three-dimensional models the downward influence of imposed perturbations in the upper stratosphere and the relation to overall dynamical sensitivity. This study will also include the effect of an imposed seasonal cycle where, even if the notion of multiple equlibria is not so applicable, there is previous evidence of dynamical sensitivity (Scott and Haynes 2002).