These changes in the Pacific Ocean and its overlying atmosphere occur in a cycle known as the El Niño–Southern Oscillation (ENSO). The atmosphere and ocean interact, reinforcing each other and creating a 'feedback loop' which amplifies small changes in the state of the ocean into an ENSO event. When it is clear that the ocean and atmosphere are fully coupled an ENSO event is considered established.

Even in a neutral state, temperatures in the Pacific Ocean vary from east to west – for example, the western Pacific 'warm pool' in the tropical Pacific has some of the warmest large-scale ocean temperatures in the world. During an ENSO event, ocean temperatures become warmer than usual or cooler than usual at different locations, which are reflected in ocean temperature gradients. The most important driver of ENSO is these temperature gradients across the Pacific, both at the surface and below the surface, particularly at the thermocline.

Cycles change every 3 to 7 years – but it’s unpredictable. And there is still debate about what actually triggers an El Niño cycle.

In general, El Niño events tend to only last for a single cycle (i.e. one year from autumn to autumn), but it is not uncommon for multi-year La Niña events to occur. For example, the 1998–2001 La Niña affected three consecutive years from autumn 1998 to autumn 2001.

The term El Niño translates from Spanish as 'the boy-child'. Peruvian fishermen originally used the term to describe the appearance, around Christmas, of a warm ocean current off the South American coast. It is now the commonly accepted term to describe the warming of the central and eastern tropical Pacific Ocean. La Niña translates as 'girl-child' and is the opposite ENSO phase to El Niño.