El Niño/La Niña-Southern Oscillation, or ENSO, is a quasi-periodic climate pattern that occurs across the tropical Pacific Ocean on average every five years, but over a period which varies from three to seven years. It is characterised by variations in the temperature of the surface of the tropical eastern Pacific Ocean - warming or cooling known as El Niño and La Niña respectively - and air surface pressure in the tropical western Pacific - the Southern Oscillation. The two variations are coupled: the warm oceanic phase, El Niño, accompanies high air surface pressure in the west Pacific, while the cold phase , La Niña, accompanies low air surface pressure in the west Pacific.[2][3] Mechanisms that cause the oscillation remain under study.

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