I went to a French Wikipedia page on circumflex. It has the advantage over the English page in that it shows how different languages treat vowels with a circumflex.
Under the French section, it says:
Dans d'autres cas, il résulte d'une voyelle double (âge pour aage, rôle pour roole) ou d'une simple évolution de la prononciation...
It would make the a in to a double (length) vowel, as vaalid. That is, the speaker would stress the (already stressed) syllable by lengthening the time that it was pronounced.
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(It may be difficult to think of a double-length vowel in English. Think of a trendy person saying, "Daahling, can you tell me the way to M&S?" Linguistically, what is happening is a doubling of the vowel length (and the deletion of a post-vocalic r)).
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There is only one vowel with a circumflex in the "Breton" section: the letter e. It says that it replaces the dipthong ae, as in encyclopaedia.