
The best I can describe is that the term "Northern America" is a noun and can be used alone as such. "North America" as well is a noun and can be used alone. But, if you make "North America" "North AmericaN", you're turning into an adjective and would expect there to be a following noun that it modifies, though you can use the term alone and it's still understood that you're describing something that is from North America. Is that clear as mud? I'll post this question on a linguistics forum though and see what comes out - it's been a long time since I've done this stuff day to day!
As far as common usage, I think 99% of us (U.S. and Canadian citizens) would say "North American", not "Northern America".