Spelling of Auntie vs Aunty?

I have always spelled the word with which I address sisters of my parents as Auntie. Of late I have noticed that just about everybody else around me seems to spell it as Aunty.

My ancestry is British but only two of my great grandparents were born there while the other six were born in South Australia.

I wondered whether the two variations in spelling were British vs American.

Can anyone enlighten me as to the origin of each?

Dictionary.com suggests the following which seemed to suggest that both are fine to use:

Word Origin and History for aunt-ie Expand n.
1787, also aunty, familiar diminutive form of aunt. As a form of kindly address to an older woman to whom one is not related, originally in southern U.S., of elderly slave women.

As stated by Etymonline auntie is originally an AmE term and aunty was just a variant. Checking with Ngram both terms were used from the late 18th century both in British and American English:

Ngram Auntie BrE vs AmE

Ngram Aunty BrE vs AmE

Auntie(n.):

• 1787, also aunty, familiar diminutive form of aunt. As a form of kindly address to an older woman to whom one is not related, originally in southern U.S., of elderly slave women.

  • The negro no longer submits with grace to be called "uncle" or "auntie" as of yore. ["Harper's Magazine," October 1883]'

Suffix -ie a variant of y:

• a noun-forming suffix with a variety of functions in contemporary English, added to monosyllabic bases to create words that are almost always informal. Its earliest use, probably still productive, was to form endearing or familiar names or common nouns from personal names, other nouns, and adjectives ( Billy; Susie; birdie; doggie; granny; sweetie; tummy). The hypocoristic feature is absent in recent coinages, however, which are simply informal and sometimes pejorative ( boonies; cabby; groupie; hippy; looie; Okie; preemie; preppy; rookie).

• A few words in which the informal character of -y , (-ie) has been lost are now standard in formal written English ( goalie; movie).

(Dictionary.reference.com)

Difference between Aunty and Auntie by diffzi.com:

The main difference between the words Aunty and Auntie are two, the first one is that Aunty is more frequently used in British English while Auntie is more frequently used in American English. The another is that Aunty is mostly used in a more formal tone while Auntie is considered a less formal word.