The Collaborative International Dictionary of English v.0.48:

Subtropical \Sub*trop"ic*al\, a.
   Nearly tropical.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subtropical
    adj 1: of or relating to or characteristic of conditions in the
           subtropics; "even near the equator vegetation at 5000 ft
           is subtropical rather than tropical" [syn: subtropical,
           subtropic, semitropical, semitropic]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "subtropical":
   aestival, blood-hot, blood-warm, calid, equatorial, genial, luke,
   lukewarm, mild, room-temperature, summery, sunny, sunshiny,
   temperate, tepid, thermal, thermic, toasty, tropical, unfrozen,
   warm, warm as toast, warmish