For centuries, Peruvian fishermen have feared the sea warming called El Niño which, every few years, around Christmas, drastically reduces their fishing catches. These El Niño events are part of a broader disruption to normal weather patterns which causes drought, flooding and hurricanes around the world.

In the tropical Pacific, ocean and atmosphere circulations are closely linked, each reacting quickly to changes in the other. Warm El Niños and cold La Niñas follow each other against the backdrop of the ocean seasons. Variable in intensity, these surface temperature and sea level anomalies in the intertropical Pacific cause major heat exchanges between the ocean and atmosphere, affecting climate worldwide.

Since the 1990s, an in situ observation system has been set up in the Pacific and new satellites have continuously scanned the global ocean. In December 1993, the Topex/Poseidon mission showed that precise satellite altimetry reveals the general features of an event several months ahead. The 1997-98 El Niño, particularly strong, was the first closely monitored event. Since then, altimetry satellites closely monitor the oceans and record the slightest variations in sea level.