The recent devastating El Niño event of 1997/98 is a fresh memory for many. It ranks as the second strongest in the past century, causing over 2,000 deaths and leaving a global damage bill of around £20 billion.

Archaeological evidence suggests that El Niños and La Niñas have been occurring for at least 15,000 years. But it is only since the mid 1970s, and the recognition of their widespread climate impacts, that these phenomena have gained worldwide attention.

The clearest sign that an El Niño event is underway is the appearance of unusually warm water, by up to 5°C, between the date line and the coasts of Ecuador and Peru. However, El Niño periods are more than just a warming of the eastern tropical Pacific. The entire tropical Pacific ocean-atmosphere system is stirred up by them.

Large scale variations in atmospheric pressure between the Pacific and Indian oceans (a Southern Oscillation) accompany El Niño. So they are also often described as being a warm ENSO (El Niño Southern Oscillation).

During the last century El Niño and La Niña events occurred in equal numbers with an average return period of about four years. They usually last about a year and peak in the northern hemisphere winter.

The El Niño phase has been in the ascendancy during the past quarter century leading some to suggest a link to global warming. However, the persistence of a moderate La Niña since August 1998 has changed this viewpoint.

The cause of El Niño and La Niña is not fully understood, but an important factor is the strength of the prevailing trade winds that blow from east to west across the equatorial Pacific.

In strong El Niño events, the trades slacken or reverse direction. The pool of surface water warmer than 28°C, normally located over the western tropical Pacific, is forced eastwards. It evaporates, resulting in drenching rains over South America (east Pacific). Meanwhile, Indonesia in the west Pacific, experiences drought conditions.

In contrast, when La Niña occurs, the south east trades strengthen and water even warmer than usual is piled up in the west Pacific. This leads to excess rains in the west and dry conditions in the east Pacific.

Related links:
El Niño and La Niña Impacts
More on La Niña
University College London: Department of Space and Climate Physics

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