I’ve often said that the evidence for actual periodic (or even pseudoperiodic) behavior in ocean cycles is sketchy at best. What are usually quoted as periods are better referred to as characteristic time scales. Furthermore, it’s all too easy to misinterpret period analysis (usually in the form of spectral analysis) even when estimating the values of, or assessing the existence of, characteristic time scales.
I don’t deny the existence of fluctuations (which I regard as a better description). Nor do I claim that they don’t show characteristic time scales — just that the evidence is often sketchy at best. As far as being actually periodic (in the sense that knowledge of the last few “cycles” enables us to make some useful prediction of the next, or the next few, “cycles”), I believe that they’re not. I could be wrong — but I’m still waiting for evidence.