Hyper-boreal is an adjective used to describe things that are related to or situated in the extreme north. The concept implies cold, snow, glaciers, and arctic regions. The antonyms of the word could be related to the opposite extreme of temperatures, locations or other elements.
One antonym for hyper-boreal could be tropical, a term associated with hot, humid, and rainforest climates. Another could be temperate, a term used for areas with moderate temperatures throughout the year. Subtropical could be another antonym, referring to warm and humid biomes located between the tropics and the temperate zones. Lastly, words like balmy, mild, or sunny could also be antonyms to hyper-boreal, conjuring notions of warm climates, sunny skies, and pleasant weather.