The effect of the 11-year solar-cycle on the temperature in the upper-stratosphere and mesosphere: Part II numerical simulations and the role of planetary waves
Résumé
Results from mechanistic model simulations have been analysed to examine the effect of the solar cycle, and in particular how the level of planetary wave activity changes the effect of the solar cycle. The model is a stratosphere and mesosphere model with detailed chemical, radiative and dynamical schemes. Planetary waves are initiated at the lowest boundary level of the model, which corresponds to the tropopause height. Model simulations have been carried out in pairs, with one simulation using solar forcing corresponding to solar minimum and the other to solar maximum. The level of lower boundary planetary wave forcing is varied between pairs of model simulations. The difference in temperature signal between the pairs of simulations is presented. The results illustrate the crucial role played by the planetary wave forcing in the solar cycle temperature signal. The solar cycle temperature signal in the tropics and subtropics is about 1 K for all values of wave forcing. However, in the extra-tropics the solar signal varies critically with wave forcing, giving a solar signal as strong as 16 K for intermediate values of wave forcing. Despite some spatial differences, the simulations with a specific wave forcing show good qualitative agreements with observational results presented in the companion paper [Keckhut et al., 2005. Journal of Atmospheric and Solar-Terrestrial Physics, submitted for publication]. Above a critical level of wave activity, the non-linear interaction with the mean flow induces a stratospheric warming and a strong temperature change. The critical wave-forcing amplitude necessary to produce such an event is very sensitive to the initial state of the atmosphere and a small change of the mean wind, due for example to an enhancement of the solar forcing, can generate a large difference in temperature, depending on the level of the wave forcing. The numerical simulations presented here suggest a mechanism by which a small change induced by the solar forcing can generate a large atmospheric response.