NOAA
Noaa: Pacific Marine Environmental Laboratory: What Is El Nino?
This page tells you what El Nino is, provides links to animations and videos on El Nino, explains how to recognize El Nino, and discusses the history of significant El Nino events.
PBS
Nova: Into Living Things
Paleoclimatologists use information from tree rings, corals and ice cores to learn more about El Nino. See if you can analyze samples of each of these types of evidence to look for past El Nino events.