SOL is set at close to the angular frequency of the average solar cycle -- 10 to 12 years in period. For a more detailed look at the QBO modulation, the analysis of comment #10 indicates that this might be closer to a 15 year period. I am simply trying to characterize the behavior in terms of other known phenomenon.
The solar cycle as it impacts the earth has 3 main periods. (1) a yearly cycle corresponding to seasons (2) a semiannual cycle as the sun crosses the equator twice a year and (3) solar activity corresponding to sunspot cycles. As the QBO is measured higher in the stratosphere, the average cycle transitions from a quasi-periodic 28 months to a strong 6 month period. How does this transition come about is the question. It may be resonantly "pumped" by a combination of these 3 known periods. That's all I was trying to do with the figure in comment #7.