WordNet records no synonym and no antonym for monsoon. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a seasonal wind in southern Asia; blows from the southwest (bringing rain) in summer and from the northeast in winter
rainy season in southern Asia when the southwestern monsoon blows, bringing heavy rains
Synonyms, antonyms and the broader and narrower relations for monsoon come from Princeton University’s WordNet. Full sources and licences.