The Collaborative International Dictionary of English v.0.48:

Sonoran \So*no"ran\, a. (Biogeography)
   Pertaining to or designating the arid division of the Austral
   zone, including the warmer parts of the western United States
   and central Mexico. It is divided into the

   Upper Sonoran, which lies next to the Transition zone, and
      the

   Lower Sonoran, next to the Tropical.
      [Webster 1913 Suppl.]