The 15th edition of The Chicago Manual of Style said that “Southern California” should have both words capitalized, but “northern California” should not. This has changed in the 16th edition.

CMOS-16 rule 8.46 says that both Southern California and Northern California are capitalized. This is not the case for any other states.

Also capitalize in these instances:
     the North, Northern, Northerner (and the South, Southern, and Southerner) in American Civil War contexts
     the Deep South
     the South of France (region)
     Southeast Asia
     North/South Africa, North/South African countries
     North/South America, the North/South American continent
     the North/South Atlantic
     the Northern/Southern hemisphere

Lowercase in these instances: 
     northern/southern (part of a country)
     a northerner/southerner (outside of Civil War contexts)
     in northern/southern Africa
     a northern/southern Atlantic route
     north/south, northern/southern, northward/southward, to the north/south (directions)