Continuing with the forest & trees analysis of ENSO, there's just an incredible amount of fluctuation detail in the time-series.
If a fit is concentrated on the daily SOI data from 2010 to 2013 (high-resolution training range shown in yellow):
Next, if that fit is relaxed and the lower-resolution monthly data from 1880-present is simultaneously fit with the 2010-2013 daily data fit:
The possibilities for cross-validation of models are endless with such a rich data set. I think I am just scratching the surface with the possibilities.