Some disambiguation and further corollaries in light of the references above.
Free Oscillation is the unforced motion of an oscillator at its natural frequency. I have been equivalently calling this Helmholtz Resonance. Damped and Forced Oscillation are other applicable parameters. Forced Oscillation can be sustained more or less harmonic (resonantly-coupled) forcing, tending to equilibrium with negative-feedback limit-cycle damping, or Forcing can be more like singular Impulses (kicks) that cause free oscillation, that then damps away.
We can heuristically predict, from applicable physics theorems, that all these modes somewhat express, by turns, in complex geophysical oscillations like ENSO-QBO.
We can heuristically predict, from applicable physics theorems, that Lunisolar Tidal Effects do not just excite and occasionally force ENSO-QBO oscillations, but just as surely sometimes damp free oscillation and trigger chaotic state-transitions.
These are definite falsifiable heuristic predictions for ENSO-QBO data analysis to validate, as the science continues to advance.