El Niño and La Niña events are a natural part of the global climate system. They occur when the Pacific Ocean and the atmosphere above it change from their neutral ('normal') state for several seasons. El Niño events are associated with a warming of the central and eastern tropical Pacific, while La Niña events are the reverse, with a sustained cooling of these same areas.
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.
Pacific Ocean – even in neutral state the Western Pacific is warm
Why are they called El Niño and La Niña?
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.
Because ENSO involves interaction between the ocean and the atmosphere – both of which play a role in reinforcing changes in each other – it is known as a coupled ocean–atmosphere phenomenon.
Thermocline comes from the Greek for 'heat slope' and is the name for the region separating warm, well-mixed surface water from cool, deep ocean water. Typically water temperatures above the thermocline are more than 25°C while those below the thermocline are 15°C or less.