We don't have synonyms for mexamerica yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are metameric, mesoamerica and hexameric.
(n) :
The southern and Central Valley portions of California as well as southern Arizona, the portion of Texas bordering on the Rio Grande, most of New Mexico, northern Mexico, and the Baja California peninsula.
(n) :
The southern and Central Valley portions of California as well as southern Arizona, the portion of Texas bordering on the Rio Grande, most of New Mexico, northern Mexico, and the Baja California peninsula.