astrophysics - Causes of hexagonal shape of Saturn's jet stream

NASA has just shown a more detailed picture of the hexagonal vortex/storm on Saturn:

http://www.ibtimes.com/nasa-releases-images-saturns-hexagon-mega-storm-may-have-been-swirling-centuries-1496218
https://en.wikipedia.org/wiki/Saturn_hexagon

Is that theoretically understood what is the cause behind this eye-catching nontrivial, regular yet not circular, shape? If so, what is the cause? I expect some explanation in terms of "nonlinear equations" of "mathematical physics" and "solitons".

P.S. (added a day after this question and the first answer was posted): On my blog where I posted the same question, people came up with some articles and phrases like "Rossby waves" and "resonance of latitude-dependence Coriolis frequency".