"Like any art, poetry is meant to communicate something between the writer and the reader. Poets must choose a path between plain telling and metaphorical showing. Tip the balance towards telling, and the poetry risks becoming banal, 'hallmark'. Tip the balance toward metaphor and the poem risks falling into individual metonymy--associations that have meaning only for the poet."
Metonymy, like metaphor is a figure of speech. But where metaphor compares, metonymy associates.
From the wikipedia article on metonymy:
"the metaphorical phrase "fishing for information", transfers the concept of fishing into a new domain. If someone is "fishing" for information, we do not imagine that he or she is anywhere near the ocean, rather we transfer elements of the action of fishing (waiting, hoping to catch something that cannot be seen) into a new domain (a conversation). Thus, metonymy works by calling up a domain of usage and an array of associations (in the example above, boats, the ocean, gathering life from the sea) whereas metaphor picks a target set of meanings and transfers them to a new domain of usage."
Both can be effective poetic tools. Both relate disparate concepts to create a third and enhanced meaning. But when metonymy employs personal associations that do not have a cultural or linguistic referent, then the reader is locked out of the life of the poem.
I am reminded of an episode of Star Trek: The Next Generation (Darmok)in which Picard and the Enterprise encounter a race of beings who speak using metaphor and metonymy. Though the 'universal translator' translates their words, their words have no intrinsic meaning for the Enterprise crew as they are entirely based on shared cultural meaning and association. (For a good example of this, there is a lexicon developed from that episode here.)
It is interesting that while the discussions of this episode describe the aliens as speaking in metaphor, in fact they are using associations rather than direct comparisons, which in my mind leans toward metonymy.
Another interesting slant on metonymy is seen in Kristina Chew's article ""Fractioned Idiom: Poetry and the Language of Autism". In it, she discusses autistic language as a highly refined, individual metonymy.
As Chew relates:
"In metonymy, one thing is related to another because those two things just so happened to occur in close succession to each other. So, for a while, "sushi" meant "bike ride" to my son Charlie because I had one day bought him sushi for lunch after he had been on a bike ride and not (metaphorically) because of a resemblance between the wheels of his bike and the seaweed-edged rounds of sushi)."
In order for Chew to make sense of Charlie's metonymic association between sushi and bicycle, she had to enter into his world and share the event that created the association in the first place. This is also the case with 'private jokes' between people, that relate individual experiences to a linguistic cue that recreates the event. My husband only has to say to me 'mushrooms' and it evokes a college experience involving a camp fire, senior week, and sneaking into Super Wegmans barefoot to buy marshmallows. The association is personal and idiosyncratic.
Poetry that relies on highly personal comparisons (metaphor) and associations (metonymy) create difficulty for the reader because he or she was not present to share the events that created the association. Thus, highly individualized language may make for an effective emotional catharsis or personalized gift, but it may not succeed as art simply because it fails to communicate.