>As I said I think it looks biannual.
With that I mean the pattern. In particular I don't share the opinion about other periods. But actually you wanted a comment on the height of the extrasystole. For this one one would probably need to look at all sorts of climate phenomena.
According to your correlation image from  interestingly ENSO precedes QBO (intuitively I could have guessed the other way around), so this could be one component