There are a few southerly species :
Jack jumper ants (Tasmania)
Clubbed Trigger ants (New Zealand)
Bullhead ants (Southern Africa)
Monomorium bidentatum (Patagonia)
Weaver ants Œcophylla (Tierra del Fuego)
So there are clearly more than one possible candidate for rafting.
It seems that ants are kind of bound by minimum temps
But that's not to say that certain species cannot collectively buck the cold
https://phys.org/news/2016-06-mountaine ... -body.html
https://biology.stackexchange.com/quest ... ts-to-cold
IMHO I think that ants are incapable of digging far into/combatting permafrost, though.
And I bet that in a place like Antarctica, no (complex) plant associations with large taprooted plants/shrubs/trees might have something to do with it as well.