Couple of recent posts where the harmonic series approximation for the ENSO forcing is reduced to a closed-form expression:

http://contextearth.com/2017/10/27/reverse-engineering-the-moons-orbit-from-enso-behavior/

http://contextearth.com/2017/11/03/approximating-the-enso-forcing-potential/

This simplification was also applied to QBO

![QBO](https://i2.wp.com/imageshack.com/a/img922/3353/w1aSa0.png)

These are timely findings, as the presentation was accepted to the AGU next month

[GC41B-1022: Biennial-Aligned Lunisolar-Forcing of ENSO: Implications for Simplified Climate Models](https://agu.confex.com/agu/fm17/meetingapp.cgi/Paper/221914) ![mtg](https://i2.wp.com/fallmeeting.agu.org/2017/files/2017/08/FM_WebsiteLogo_483x177-1.jpg?w=300)

Check the sub-title :)