What are the principal differences between the Southern Baptist and American Baptist denominations in the US?
The Southern Baptist Convention is a formal denomination. They have a web site.
So is the American Baptist Convention. Is that who you mean when you say Northen Baptists? There's also the National Baptist Convention which happens to be a primarily African-American association. ABC and NBC also have web sites.
Please keep in mind that the Baptist traditions are free church traditions. They got started in the USA when Roger Williams bailed out of theocratic Massachusetts to found Rhode Island with a driving principle of soul freedom.
Free church? Specifically, each congregation establishes their own covenant (statement of beliefs and practices). Therefore, generalizing is quite difficult, and may lead you to unfaithful conclusions.
Any congregation I've ever heard of is honored to talk to a visitor about their covenant. So ask!
Now to your question: Here is one little tidbit of difference between the SBC and the ABC. A seminary teacher of mine, Prof. Mark Heim, is a member of the American Baptist Convention, and has represented them on the World Council of Churches Faith and Order Commission.
He taught very clearly that Holy Baptism is a universal rite for the whole church (across denominations, traditions, languages, and customs) according to Ephesians 4:4-6 ("one Lord, one faith, one baptism, one God and Father of all, who is above all and through all and in all.") He taught that when one congregation honors baptism administered in another congregation it's good for Christian unity. And the ABC suggests that for their congregations.
On the other hand, the SBC, at least according to an answer here on this site, recommends that congregations sometimes rebaptize. See Do any major denominations refuse to recognize Holy Baptism in other denominations
So, that is a difference.