Dynamical response to the solar cycle
The dynamical impact of the 11-year solar cycle is investigated with the focus on the stratopause region where solar ultraviolet heating is greatest. The most important variation in solar forcing longer than the diurnal cycle is the annual cycle. Thus the climatological features of the zonal wind variation associated with the annual cycle were first studied to characterize the basic features of the atmosphere's dynamical response to changes in solar radiative forcing. The 11-year solar cycle effect was then investigated. The results of the analysis suggest that in a climatological mean state the stratopause circulation evolves from a radiatively controlled state to one dynamically controlled during winter in both hemispheres. The transition period is characterized by a poleward shift of the westerly jet. The solar cycle effect appears as a change in the balance between the radiatively and dynamically controlled states. The radiatively controlled state lasts longer during the solar maximum phase, and the stratopause subtropical jet reaches a higher speed. The large dynamical response to relatively weak radiative forcing may be understood by the bimodal nature of the winter atmosphere due to interaction with meridionaly propagating planetary waves and zonal mean zonal winds. It is suggested that the solar influence produced in the upper stratosphere and stratopause region is transmitted to the lower stratosphere through (1) modulation of the internal mode of variation in the polar night jet and (2) a change in the Brewer-Dobson circulation. The first process is significant in the middle and high latitudes, whereas the latter is prominent in the equatorial region.