What is the meaning of "within-season variation"?
"Within-season variation" may be referring to one of two things (depending on the context):


More commonly: the variation of a variable within a given season (i.e., between months or days within that season).


Öst (1999) demonstrates an example of this use (in the context of breeding season). 

Less commonly: the variation of a variable between the same season of different (often consecutive) years. Though this would more often be termed "between years". This is as opposed to "between seasons" which would be be between two different seasons of the same year.