southerly vs southern

southerly is a noun but southern is not a noun.

Word	Noun	Adjective	Verb	Adverb
southerly	Yes	Yes	No	Yes
southern	No	Yes	No	No

As adjectives, southerly and southern are synonyms defined as:

As adjectives, southerly and southern are synonyms defined as:

southerly (noun)	southern (noun)
a wind from the south

southerly (adjective)	southern (adjective)
from the south; used especially of wind	situated in or coming from regions of the south
situated in or oriented toward the south	in or characteristic of a region of the United States south of (approximately) the Mason-Dixon line
	from the south; used especially of wind
	situated in or oriented toward the south

southerly (adverb)	southern (adverb)
from the south
toward the south