Pepsi has chosen TBWAChiatDay New York as lead creative agency for the Mountain Dew brand following a closed, Omnicom-only review. “We enjoyed a long and fruitful relationship with BBDO New York and are grateful for the partnership,” Mountain Dew vice president of marketing Nicole Portwood said in a statement confirming the switch. “We’re excited about our new chapter with TBWAChiatDay New York and look forward to continuing our longstanding partnership with Omnicom.” According to parties with knowledge of the matter,…