UK:*UK and possibly other pronunciationsUK and possibly other pronunciationshwɛərəˈpɒn/

US:USA pronuncation: IPAUSA pronuncation: IPA/ˌhwɛrəˈpɑn, -ˈpɔn, ˌwɛr-; ˈhwɛrəˌpɑn, -ˌpɔn, ˈwɛr-/

US:USA pronunciation: respellingUSA pronunciation: respelling(hwâr′ə pon, -pôn, wâr′-; hwârə pon′, -pôn′, wâr-)

WordReference Random House Learner's Dictionary of American English © 2020
where•up•on /ˌhwɛrəˈpɑn, -ˈpɔn, ˌwɛr-; ˈhwɛrəˌpɑn, -ˌpɔn, ˈwɛr-/USA pronunciation   conj. 
  1. upon what or which.
  2. at or after which:He'll surely try to get in touch with her, whereupon we'll spring the trap.

WordReference Random House Unabridged Dictionary of American English © 2020
where•up•on  (hwâr′ə pon, -pôn, wâr′-; hwârə pon′, -pôn′, wâr-),USA pronunciation conj. 
  1. upon what or upon which.
  2. at or after which.
  3. [Archaic.]upon what?
  • Middle English wherupon. See where, upon 1300–50

Collins Concise English Dictionary © HarperCollins Publishers::
whereupon /ˌwɛərəˈpɒn/ sentence connector
  1. at which; at which point; upon which
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