It's like this. In the following DiffEq profile, both the periodic impulse (LHS) and the response (RHS) is provided:
If you don't have the impulse and only have the response, one can try to guess what the impulse is. For ENSO, I had to guess what it is because the geniuses at NOAA filtered it out. No matter ... since when I added the filtered impulse back in (found as an analysis artifact in the "ensostuff" directory), it *exactly* matched my guess anyways.
The scientists at NASA JPL warn about doing this kind of thing. [Perigaud et al](https://archimer.ifremer.fr/doc/00101/21178/18795.pdf) have pointed out how reckless it is to remove what are considered errors (or nuisance parameters) in time-series by assuming that they relate to known tidal or seasonal factors and so can be safely filtered out and ignored.
> " ... real-time Lunisolar tidal noise ... " "... and the tidal noise they are subject to ..."
"One man's noise is another man's signal", which was a [well-known saying from the space program](https://en.wikipedia.org/wiki/Edward_Ng) LOL