> Like I said, there is no way to test for how well it will extrapolate...
Of course there is. It is standard practice in machine learning, see [cross-validation](https://en.wikipedia.org/wiki/Cross-validation_%28statistics%29)
> The author claims that QBO cycles are either 24, 30, or 36 months long...
Sounds rather like [what Nad said](https://forum.azimuthproject.org/discussion/comment/14533/#Comment_14533) a while ago.
> well the QBO oscillations have of course a little fuzzy frequency component, but I wouldn't call this "frequency modulation."
And strictly speaking the QBO is even not really periodic, but only somewhat and the amplitude goes also quite wild.
What I mean is, as said here that it looks as if the signal is mostly having a strict two year period
(i.e. I mean by that f(t) comparatively small) but once in a while not - i.e. it gets out of sync.
Moreover it looks as if it is "forced back" into the biannual rythmn.
I could imagine that this "out of sync" (disturbations, breaks) and forced back behaviour in an oscillation
is rather well distinguishable from a more random behaviour via a fourier transform but may be not and even
if this would be so I don't know how the corresponding typical forms would look like.