the dominant factor is the sun. the vortex shrinks in the summer and expands in the winter, reacting to the amount of sunlight hitting the earth. if we were to turn the sun off, the polar vortex would sweep over the earth, and we'd be plunged into a snowball earth scenario. and, if we were to turn the sun up enough then the vortex would disappear altogether. the expansion and contraction of the vortex is strongly correlated with the solar cycles. we're currently in a period of decreased solar activity. this is being counter-acted, however, by warming oceans due to global warming.

(the idea that this is being caused by warming is grasping at straws, and really rather outrageous given how entrenched the understanding of the vortex is. see mike lockwood, and in fact michael mann, for a better explanation of the solar effects.)

www.theweathernetwork.com/news/articles/polar-vortex-explainer/63115/