Here's why:
* "Northern" is an adjective, not a preposition. Adjectives describe nouns, while prepositions show the relationship between words.
* Prepositions are needed to show location. You need a preposition like "north of," "to the north of," or "in the north of" to indicate a location relative to the city.
Correct examples:
* North of the city: This is the most common and straightforward way to express the location.
* To the north of the city: This emphasizes the direction.
* In the north of the city: This emphasizes being within the northern part of the city.
Instead of "northern of," use one of these prepositions and the correct adjective form of "northern," which is "north."