I would have thought it the opposite (ambiance > ambience).
I would have thought “ambience” on the rise in particular the past few years.
Maybe Google’s Ngram is looking at different sources than I am.
If these two are on the decline, what is on the rise?
Braulio Agnese noted for me the relative prominence is flipped in Google Trends.
A friend, Jimmy Kipple, suggested “chill,” and that seems like a good bet, even more right than I’d imagined.
Chill is the word (is the word, is the word that you heard)
It’s got a groove, it’s got a meaning
Chill is the time, is the place, is the motion
Chill is the way we are feeling
Another friend, Robert Boyd, had a good call: “vibe.” On the one hand, “vibe” is lower than “ambient” or “chill.” On the other, “vibe” is a much closer synonym for “ambience” and “ambiance,” and its rise aligns with their decline. (Vibe’s overall rise in the Ngram, it should be noted, begins almost exactly with the launch, in 1993, of the magazine Vibe.)