Diphthongisation in varieties of English

Question

Consider the vowel in a word like "know". The way I pronounce it sounds maybe like /nøʉ/ to me. But other Brits oftentimes think I have a foreign accent, so I don't know about that. And I am sure I've heard people say /no:/ as well.
But what I'm interested in knowing about is the pronunciation /nuɔ/.

It seems to appear in Jamaica (I say so because of Lee Perry's pronunciations in an old recording of the song "I Man Free"). And the Wikipedia page on Jamaican Patois gives this example:

/if kau no did nuo au im tɹuotuol tan im udn tʃaans pieɹsiid/ 
('If the cow knew that his throat wasn't capable of swallowing a pear seed, he wouldn't have swallowed it')

(And by the way I thought it notable that know and no are not homophonous in this variety of English).

I will make another assertion, but this one is not backed up by the Wikipedia page on Mackem or Geordie: The same sound change has happened in the North East of England, and appears to be accompanied by a similar process that's /eɪ/ → /iə̯/ here and in Jamaica alike.

By my intuition, the "semivowel half" of a diphthong is usually a high vowel, like /ɪ̯/ or /ʊ̯/. By my intuition, diphthongs that involve a glidey thing that's qualitatively like /ɔ/ or /ə/ are cross-linguistically very rare. So has there been some kind of analysis on how and why this has turned up around twice, and in at least two varieties of English separately? Or have I misunderstood something?

Answer 1

This pathway of falling diphthong > (long) monophthong > rising diphthong has happened several times in linguistic history. Cf. Classical Latin caelum /ˈkae̯.lum/ with falling diphthong, Vulgar Latin */ˈkɛ.lum/ with monophthong, Castilian Spanish cielo /ˈθjelo/ with glide or rising diphthong.

The rising diphthongs [uo] and [ie] are a relatively conspicuous part of the phonology of basilectal Jamaican Creole. In Standard Jamaican English, these tend to be long monophthongs [o:] and [e:], corresponding to the Wells lexical sets GOAT and FACE respectively.

Your intuition about Geordie is borne out by this presentation, given at a conference in 2015. There is apparently a split based on whether the syllable is closed or not. Although /ə/ as the nucleus of the diphthong might be rare, it's not unheard of; old-school (pre-1970s) Received Pronunciation has an abundance of "centering" diphthongs, albeit falling rather than rising, e.g. CURE /kjʊə̯/ (analyses as a triphthong aside). An example of a rising diphthong onto /ə/ is in Central Catalan (e.g. of Barcelona) - qüestió is pronounced /kwəs.tiˈo/, with a glide onto /ə/ in the initial syllable.