in sunny Jeju vs. in a sunny Jeju

in sunny Jeju vs. in a sunny Jeju

"in sunny Jeju" seems to me that it's possible for Jeju to be naturally sunny.
"in a sunny Jeju" implies that it's not possible for jeju to be naturally sunny but it happened one day.
Note that a/an can be used to call attention to something really unusual.
Another example)
We held a big celebration in bustling Chicago. : Chicago is a big city and always bustling.
It was a bustling Podunk the night of our celebartion: Podunk is a very small and quiet town, but the night the town was very bustling.

 

 

출처: http://blog.daum.net/bizncs/15932789