> I've been reading about the [Walker circulation](http://en.wikipedia.org/wiki/Walker_circulation), where it says "When the Walker circulation weakens or reverses, an El Niño results".
Right! When this westward-blowing wind slows down, hot water that has piled up on the west side of the Pacific starts moving east, in blobs. Apparently if enough of these blobs move east we get a El Niño. You can see several of them moving east this year:
* [ScienceCasts: El Niño - Is 2014 the New 1997?](http://youtu.be/zaxPwASV2kY).
Everyone should watch this nice short movie!
> Does that sort of change - a change in the number of convection cells in a volume of fluid - count as a 2nd-order phase transition?
I don't know. I'd be a bit surprised if it did. 2-point functions go wild during phase transitions, but during a 2nd-order phase transition they become scale-invariant.
The melting of ice is a first-order phase transition: right at the brink of melting we expect long-range correlations between whether ice is melted here and whether it's melted somewhere else. The point where the difference between the liquid and gas phases ceases to exist is a second-order phase transition. Near this point we see droplets of water floating in water vapor, and the situation becomes closer and closer to _scale-invariant_ near the critical point, meaning that we see droplets of all sizes. This is visible as [critical opalescence](http://en.wikipedia.org/wiki/Critical_opalescence).
If a second-order phase transition were occurring near the onset of an El Niño, we'd see some scale-invariant phenomena. For example: blobs of warm water of all sizes, big and small, starting to move east. Or something like that. 2-point functions would obey power laws.
This would be fun to look for, but I'm not betting on it. I'm just hoping that that some sort of longer-range correlations arise, as the whole system "comes to a consensus" about whether there will be an El Nño.