The reason one uses the Draconic month and not the Tropical month for explaining QBO is that the Tropical month will only give the return period of the phase of the moon for a *particular longitudinal location*. But the QBO is a longitudinally invariant behavior, as it completely encircles the equator with a largely in-phase wind at any instant of time. So the Draconic month is the cycle that gives the maximum excursions between latitudunal nodes of the moon -- independent of longitude -- which is the necessary cyclic forcing for stimulating a cross-wind at the equator (the curl term following Laplace's tidal equations [described here](http://contextearth.com/2016/07/04/alternate-simplification-of-qbo-from-laplaces-tidal-equations/) ).

The QBO model is a perfect blend of intuitive physical reasoning and a precise mathematical formulation, just like the [theory of ocean tides](https://en.wikipedia.org/wiki/Theory_of_tides). I don't understand why this has been missed by Lindzen and his followers over the years. All I know is that it will be difficult to unwind a "just-so" explanation offered up by Lindzen for QBO that has been reinforced into a questionable consensus over the course of time.

> "What is a statistically exact value?"

Once in a blue moon statistically occurs every 2.715 years, with the Tropical month beating with a yearly cycle. This uses the Tropical month because it is a *human observable* event, meaning that it occurs at the same longitude over time. Same reason that Easter uses the Tropical month as it is an observation of the Full Moon in the Middle East region, not at an arbitrary longitude.