Currently Elasticsearch does not support cross-region/datacenter replication because the replication that is done at shard level was not designed for high latency networks. Which means that a cluster that spans multiple AWS regions will suffer from availability issues (a cluster with a master node in one region and data nodes in another one won't last much long)
The recommendation is that you should maintain separate clusters per AWS regions and replicate data before indexing.
Native cross datacenter replication is on the roadmap, though. See https://www.elastic.co/blog/elasticsearch-sequence-ids-6-0