It is generally said that El Niño causes the warm waters of the tropical west Pacific ("Warm Pool") to flow towards the coasts of Ecuador and Peru. This leads to a shift in the ocean's thermal content from west to east, which modifies ocean-atmosphere heat exchanges and therefore interferes with the climate of the entire planet.

Although it was believed that El Niño was essentially defined by this movement, it has recently been discovered that it can also appear as a localised warming of the eastern part of the warm pool. This form of El Niño, known as the Warm Pool El Niño or El Niño Modoki (modoki is Japanese for "similar, but different"), although on a smaller scale, distinguishes itself from its sibling through its specific impact on global atmospheric circulation. In particular it causes severe droughts in Australia and India as it disrupts the monsoon. An analysis of the eleven IPCC (Intergovernmental Panel on Climate Change) models has shown that global warming will lead to more frequent El Niño Modoki events.

Observations over the last few decades have indicated that the relationship between the number of El Niño Modoki and classical El Niño events has intensified greatly. The phenomenon observed in 2004 was an El Niño Modoki. The year 2009 in the tropical Pacific has also seen climate anomalies corresponding to this new type of El Niño. Observing the tropical Pacific with altimetry, as well as with various types of sensors from in situ networks or satellites, and forecasting using models such as Mercator Ocean should improve our understanding of this phenomenon and help us predict its consequences, in order to be better prepared for them.