Asked by: Liz Barrett, Lancaster
This anomalous warming of the central Pacific causes bizarre weather events in Peru around Christmas. The effects are felt much further afield, but it has proved hard to predict their appearance.
Records show that El Niño occurs at intervals of between two and seven years, but the strength of the El Niño event also varies. There appears to be an El Niño developing in 2009 but it is still too early to say how strong it will be.
Robert is a science writer and visiting professor of science at Aston University.