There are 3 interpretations of “yeah, right” in American English, but I only have two of them in my dialect (I’m originally from California). I’m in my late 30s and I hear this particular version from younger folks a lot (I can imagine my teenage niece saying it this way), but I’ve also heard it from a 30-ish father of 3, so I’m not sure what generation it’s most closely associated with (perhaps I just missed it). The three interpretations I know of are as follows:
1) Normal (factual agreement): yeah right = ‘yes, that is correct’
2) Sarcastic (opposite meaning): yeah right = ‘no way in hell’
3) Back-channel (sentiment agreement): yeah right = ‘mm-hmm’
Thanks to the influence of Seinfeld and Friends throughout the 90s, (2) sarcastic is probably the default use these days, but it is the 3rd use that I don’t have in my dialect. I would say that (3) is in the same class of back-channel expressions as “you go girl!”
These three interpretations all involve different prosodic realizations; roughly, they have different tones. I’ll dig deep into my past when I studied the tone languages Mandarin Chinese and Cantonese (12 years ago) and when I actually took a phonetics course (10 years ago) to see if I can offer a plausible hypothesis about the F0 differences.
1) Normal: yeah = falling mid-low; right = falling mid-low
2) Sarcastic: yeah = rising low-high; right = rising low-high
3) Back-channel: yeah = steady mid-mid; right = rising low-high
I have little confidence in my intuitions about the prosodic properties of (1), but I feel (2) and (3) are a pretty good guess.
BTW, I happen to run across this paper by Joseph Tepperman et al. from USC: “YEAH RIGHT”: SARCASM RECOGNITION FOR SPOKEN DIALOGUE SYSTEMS. I haven’t read it, but it seems somewhat relevant to my point: “This paper presents some experiments toward sarcasm recognition using prosodic, spectral, and contextual cues.”