It's the shift in temp differences due to global warming that is sending the polar vortex on a loop more than «historically normal» would have it. (realclimate.org)
The mechanisms are different, as well as the magnitudes (like temp differences) and consequently time scales. (skepticalscience.com)
I'm not sure how well the hemispheric temp difference is understood quantitatively in light of these and other considerations, but it seems that you're jumping to conclusions just a little bit too fast. (ourchangingclimate.wordpress.com)