The most obvious autumn indicators in Persuasion come in chapter 10 on a "very fine November day" and the Long Walk.

" the view of the last smiles of the year upon the tawny leaves and withered hedges… autumn, that season of peculiar and inexhaustible influence on the mind of taste and tenderness…"

I like the idea here that autumn has more power over those people of "taste and tenderness" than those without and the further observation that " every poet, worthy of being read" has written of autumn, and she comforts herself with that poetry.

After Anne overhears Wentworth and Louisa's revealing chat

" The sweet scenes of autumn were for a while put by, unless some tender sonnet, fraught with the apt analogy of the declining year, with declining happiness, and the images of youth, and hope, and spring, all gone together"

Our poor Anne has her physical autumn shoved into her face by Wentworth, via the ever-helpful Mary:

"Altered beyond his knowledge!" Anne fully submitted, in silent, deep mortification. Doubtless it was so… the years which had destroyed her youth and bloom had only given him a more glowing, manly, open look, in no respect lessening his personal advantages." (ch. 7)

And plummets straight into winter:

"Now they were as strangers; nay, worse than strangers, for they could never become acquainted. It was a perpetual estrangement… His cold politeness, his ceremonious grace, were worse than any thing." (ch. 8)