This shows how tricky it is to determine the underlying periodicity in ENSO

![100year](http://imageshack.com/a/img537/1244/0sAMdH.gif)

I did an initial count of identifying peaks and got 42 in a 100 year interval. This comes out to 2.381 years, which is close to 2.368 for the aliased draconic cycle. But then I notice a prominent shoulder at the question mark and think that the count should be 43 peaks. That would put it at 2.326 years per cycle, which is closer to the nominal period of 2.33 of the QBO.

This NASA page shows the variability in the lunar months
http://eclipse.gsfc.nasa.gov/SEhelp/moonorbit.html

Synodic month variability

![syn](http://eclipse.gsfc.nasa.gov/SEhelp/image/Fig4-3b.png)

Draconic month variability

![dra](http://eclipse.gsfc.nasa.gov/SEhelp/image/Fig4-12b.png)


Note the bimodal shape. I think this is due to the PDF of an Sine function modulation being bimodal (i.e. inverse function is an ArcSin).

![arcsin](https://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Arcsin_density.svg/350px-Arcsin_density.svg.png)

The bimodality may impact ENSO in the synodic month more than the draconic, if the fundamental period wants to lock into the aligned 2.3333 year groove of QBO. The satellites in comment #21 around 2.75 years correspond to approximately a +/-1 hour change in the synodic 29 day 12 hour 44 min cycle.