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Pulling together various time-series of historical ENSO Proxy records.
The following link is a post on modeling the McGregor Unified ENSO Proxy reconstruction
The following is an example of a Mathieu equation fit (via Mathematica) to the ENSO proxy over a span of 300+ years
$ y''(t) + b \cdot y'(t) + [a+q sin(\omega t)]y(t) = k \cdot sin( \nu t) + c_0 +c_1 \cdot t $