I think the biggest factor differentiating the climate on the halves of the globe is that there's more landmass on the upper* bit of the Earth, so my first guess would be that the difference is tied to this overall difference.
Because co2 is not evenly distributed with most polluters concentrated in northern hemisphere and prevailing wind direction to be from equator to poles.
Any air moving towards the poles must necessarily be balanced by air moving away from the poles. This manifests as "Hadley cells" and other similar wind patterns.
This suggests that when the ice melts the darker water absorbs more energy from the light, and then warmer water freezes less resulting in a permanently warmer arctic.
*Comment by the North Hemisphere Gang