ENSO and its core coupled dynamics are squarely in the tropics (approx 15S-15N) of the Pacific Ocean.  Certainly there can be impacts outside of that region... like SSTs extending into the southeast Pacific, but that is not a pre-requisite for ENSO to occur.  You may like Emily's blog post on various patterns that may precede ENSO itself... this is something we keep an eye on as well.  https://www.climate.gov/news-features/blogs/enso/visit-zoo-climate-patterns-can-precede-enso