Pronunciation symbols and variants for the vowel sounds in "tot" and "taught"

An excellent question about the pronunciation symbols in the Learner's Dictionary comes from weltall, who observes that the pronunciation symbols at Merriam-Webster's Online Dictionary are different from those here at Merriam-Webster's Learner's Dictionary. The answer involves details about regional pronunciation of English in the United States.

Merriam-Webster's pronunciation editor Joshua Guenter responds:

Here’s an easy way to think about this: do you, in your personal speech, make a distinction in the pronunciations of the words tot and taught? Broadly speaking, people in the Eastern United States do make a distinction, while people in the Western United States, parts of the Northeastern U.S., and Canada do not. For those that make a distinction, the vowel in tot is /ä/ and the vowel in taught is /ȯ/, using the transcriptions that we have in Merriam-Webster’s Online Dictionary (based on Merriam-Webster's Collegiate Dictionary, Eleventh Edition).

These vowels would be written as /α:/ and /ɔ:/ in the International Phonetic Alphabet (IPA). In the Learner’s Dictionary, it was decided not to indicate the contrast between words like tot and taught, since this contrast isn’t made by a large number of speakers, and isn’t necessary for non-native speakers to learn. Hence, in Merriam-Webster’s Learner’s Dictionary, we transcribe all such words with /α:/