Use the adjective hibernal to describe something that has to do with winter, like a grizzly bear's cozy hibernal cave.
You can talk about anything wintry as being hibernal — like the snow-covered hibernal woods — but it most often describes an animal that hibernates during the coldest months. For example, a chipmunk's long, deep winter nap can be called a hibernal sleep. The root of hibernal is the Latin word hibernalis, which means "wintry," from the root hiems, "winter."