How to perform sort on a aggregated field in ES?

I'm using ES2.1.0. I know that we can sort results of a query based on any field of the index. My use case is to perform an alphabetical sort on a field which is not available in the backend, but is calculated on the fly based on the aggregations of selected fields. This field calculates percentage based on two fields of an index. Is it possible to sort ES query response based on a calculated field?