This paper [1] has an interesting observation relating QBO and ENSO.


[1]B. F. Chao and I. Naito, “Wavelet analysis provides a new tool for studying Earth’s rotation,” Eos, Transactions American Geophysical Union, vol. 76, no. 16, pp. 161–165, 1995.

The phase relationship is obvious when the 2nd-derivative of ENSO (top) is compared to QBO (bottom). The first vertical red line in the figure connects the two. At the first turquoise region, you can see a phase shift in the waveform, after which the phase flips by 180 degrees with respect to the model reference. It is then in-phase with the QBO (second vertical red line) for approximately 15 years. Then at the second turquoise region, the phase shifts and then the data reverts to following the model reference, which is o.o.p. with the QBO.


The model reference was generated by fitting only data from 1880 to 1980 -- anything subsequent to 1980 is extrapolated as a continuation of the sinusoidal forcing functions.

The working explanation for this behavior is that the standing wave in the Pacific Ocean which manifests as ENSO is metastable with respect to its initial excursion. So when the standing wave crosses zero on its way to changing polarity, it transiently has no inertia and is therefore sensitive to a forcing that could reverse its intended direction. In this case, the hypothesis is that some sort of climate and/or geophysical shift occurred around 1980 to cause the reversal. This was perhaps not the lowest energy configuration, however, and it then reversed direction back to its original phase by 1996.