The effect of the 11-year solar cycle modulation on the Southern Annular Mode (SAM) in the Southern Hemisphere is examined through analysis of observational data from 1968 to 2001. It is found that the year-to-year variability of the October-November mean SAM differs significantly according to the solar activity. In high solar activity years, the SAM signal extends to the upper stratosphere during October to December and activity in the troposphere lasts until autumn, whereas in low solar activity years, the SAM signal is confined almost inside the troposphere from October to December and it disappears by January. This situation is very similar to that observed for the modulation of the winter mean North Atlantic Oscillation in the Northern Hemisphere. An important role of ozone on the solar cycle modulation of the SAM is suggested.
Next, using a coupled chemistry-climate model, we simulated the structural modulation of the Southern Annular Mode (SAM) caused by the change in ultraviolet radiation (UV) associated with the 11-year solar cycle. In an enhanced UV run, the SAM signal in late winter extended to the upper stratosphere and persisted until the following autumn. In a reduced UV run, the signal was mostly confined to the troposphere and disappeared very quickly during the following summer. This greater persistence in an enhanced UV run is connected to the formation of an ozone anomaly in the polar lower stratosphere, produced by the modulation of the Brewer-Dobson circulation in late winter.
Furthermore, the effect of the 11-year solar cycle on the troposphere-stratosphere (TS) coupling in the southern hemisphere (SH) late winter/spring is examined through the analysis of observations and simulations with a chemistry-climate model. It is found that the TS coupling in the SH late winter/spring is significantly modified according to the solar cycle; the dynamical coupling between the troposphere and stratosphere becomes stronger with the increasing solar activity. Such modulation of the strength of the TS coupling is found to be the source of the solar-cycle modulation of the annular mode in late winter/spring.
|Figure 3, Scatter plots indicating the relationship between the correlation between the November-mean 30-hPa and 850-hPa SAM indices, and mean F10.7 index in the SH. The horizontal bar indicates the range of F10.7 indices of each winter for the calculation. (From Kuroda et al., 2007)|