The response of the middle atmosphere to external perturbations: A numerical study using an interactive two-dimensional model.

Abstract

A coupled chemical-dynamical-radiative 2-D model that has been extended up to the lower thermosphere is used to investigate the response of the middle atmosphere to external perturbations. Three types of external perturbations on the middle atmosphere are examined: thermal and mechanical forcing perturbation, ozone catalytic species perturbation, and the 11-year solar flux variability. We are particularly interested in the role of feedback in the atmospheric response to perturbations. In the stratosphere, the response to thermal forcing is primarily through radiative adjustment. However, in the summer stratosphere, a significant portion of the response occurs through the meridional circulation driven by gravity wave forcing in the mesosphere. In the mesosphere, the gravity wave forcing provides the dominant mechanism for the circulation response to thermal forcing changes. Temperature feedback is the most effective feedback mechanism on ozone loss. Throughout most of the sunlit part of the middle atmosphere, temperature has a negative feedback effect on ozone. An exception to this is in the lower polar stratosphere at late winter and early spring, where the temperature decrease enhances the ozone depletion process. During the polar night, circulation and eddy diffusion feedback have the largest impact on ozone. Transport mechanism has a much larger effect on ozone during the polar night. A small change in the residual circulation is shown to greatly affect the ozone concentration in the upper stratospheric polar night region. As a result, mesospheric wave activity may have a significant impact on the ozone depletion phenomenon in the lower stratosphere. Despite the various feedback mechanisms inherent in the model, imposing the solar flux variation generates only a weak response from the stratosphere. The predicted response of the model to long-term solar variability is substantially smaller than those predicted by previous studies. However, the downward transport of high odd-nitrogen content air at solar-maximum condition during the polar night may significantly increase the stratospheric ozone response to long-term solar variability. (Abstract shortened by UMI.).