Thought to comment on what a long strange trip it's been. The journey to modeling ENSO and QBO has been circuitous and then essentially doubled back to the most basic kind of forcing and the simplest toy differential equations.
The ENSO behavior is modeled as 2 lunar tidal signals and an annual forcing impulse applied to a delay differential equation of 1 year delay. Could have started with this premise from day one, but nothing in the research literature indicated lunar forcing had any effect on ENSO.
Same goes for QBO except that it is essentially a single lunar tidal signal and a bi-annual seasonal forcing signal - one impulse per nodal crossing. Lindzen had considered lunar forcing early but apparently couldn't find any correlation and that's why no one followed up there.
Looking back my first blog post on this topic was early 2014, so it's been almost 4 years of spare-time effort. And even though this was anticipated to be a software coding project, the model is simple enough to express on a spreadsheet without the need for any macros or scripts except for a standard Solver plugin. It's essentially a little more complex than a basic tidal analysis program.