Dara, Thanks for the sanity check.

I am trying to understand the significance of the two-year differencing.

A two year delay differencing can act as a very granular derivative, so it may be a measure of the speed at which the sea-level is rising in the harbor.

Or it could be an indicator of a previous value that has reflected back.

Timely that Blake Pollard wrote [the blog post](http://johncarlosbaez.wordpress.com/2014/09/16/exploring-climate-data-part-2/) with the animated GIF of SST, which is a result of the water volume instability.

![animated](http://math.ucr.edu/home/baez/ecological/el_nino/1976-1978_equatorial_surface_temperature_animation_pollard.gif)

Contrast that with a dynamic simulation of sloshing -- be patient with this one as it builds up over time. This is a solution to a forced Mathieu equation [1]

![sloshing](http://imagizer.imageshack.us/a/img823/6842/6fd5.gif)



[1] S. S. Kolukula and P. Chellapandi, “Finite Element Simulation of Dynamic Stability of plane free-surface of a liquid under vertical excitation.”