It means almost Arctic, but not quite. Having characteristics that are almost like the Arctic but not quite.
Other similar examples of the same usage:
Subaudible (softer than audible)
Subterranean (below ground but not too far)
Subliminal (too quick to see)
Subhuman (not quite human)
SubSahara (adjacent to the Sahara Desert)
Subcutaneous (right under the skin)
Subtropical (almost tropical climate)
Subordinate (not quite the highest person in the organization)
