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The ENSO phenomena is thought to be a coupled interaction between the Pacific Ocean and the atmosphere along with potentially other geophysical effects.
My idea is that resonances cascade starting with the strongest biennial and annual signals. These strong forcings then couple to the most compliant medium -- the low-density fluid of the stratosphere -- and create the first resonance, that of the QBO. And then this resonance couples to the higher inertial and less compliant medium of the ocean, and thus creating the ENSO resonance.
The figure below shows a simple model of a SemiAnnual Oscillation, found in the very-thin upper stratosphere, and how that transforms into a Quasi-biennial oscillation found in the lower stratosphere.
At the highest altitude the response function remains the forcing but as it moves lower, the resonance gradually takes over as the fluid response.
The contour is slanted as I added a linear phase shift as the waveform moves down the atmosphere.
That's the conceptual model that is trying to capture the following observed behavior.