The model I have for ENSO is a Mathieu equation for sloshing, which is analogous to the inverted pendulum on a moving cart experiment. As long as the cart shows a specific back-and-forth oscillation (or an up-and-down -- see below), the pendulum can stably invert.

So check out this cool video related to the inverted pendulum on a cart -- the "rolling" inverted pendulum. Last year Brian Josephson made a YouTube video where he demonstrates how the inverted pendulum property manifested itself in his kitchen !

No wonder he won a Nobel prize -- he is just a plainly intellectually curious fellow.

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This is the up-and-down cyclically forced inverted pendulum

Again, it uses a Mathieu formulation as described in the video summary:

>"This inverted pendulum was realized by only oscillating its pivot in 58Hz. It can be done without feedback control.
You can find the principle explained in some textbooks about non-linear dynamics. This equation of motion is classfied as Mathieu equation. By solving this equation, you can find the stable condition to keep an inverted pendulum upright.