We have the words "septentrional" and "meridional". "Boreal" and "austral" wouldn't be good fits as they relate to the north of the Northern Hemisphere and the southern parts of the Southern Hemisphere. Then again, "septentrional" and "meridional" can refer to the northern or southern part of any country or region respectively, irrespective of the hemisphere where they are located. Surely both words are dated and we would rather use "northern" and "southern".
Example sentences from EOD
- "the meridional leg of the journey"
Other examples from the web:
"in meridional regions of the Qinghai-Xizang Plateau (China), from low latitudes to high latitudes, there is a phenomenon of oscillation of atmospheric low frequency wave with 30-40 day period."
"Prenatal exposure to organochlorines and heavy metals in the Arctic largely exceeds that encountered in meridional regions of the province of Quebec."
"By far the most disturbing event of the meridional leg of the journey was..."
The Latin word septentrional, as mentioned by Sven Yargs, would be the perfect opposite for meridional except that it is seldom used in English. It's commonly used in Romance Languages, though.
Addendum: In BrE "meridional in temperament" relates to "Southern Europe" or Mediterranean, especially the South of France.