The world's weather is intimately tied to the ocean currents, as is seen in the phenomenon known as the El Niño-Southern Oscillation. The term El Niño refers to a body of warm water that gradually builds up in the western Pacific and which, in some years, travels back across the ocean towards America. In years when an abnormally large El Niño occurs, surface currents and airflows in the equatorial region of the Pacific temporarily reverse direction, producing dramatic changes in the region's weather patterns.

El Niños were first recognised by fishermen along the Pacific coast of South America. There, the main effect of the arrival of El Niño is a rapid warming of surface waters, leading to a sudden decline in fish stocks. In normal years this lasts for some two to three months. The name El Niño, 'Boy Child' in Spanish, refers to the fact that the phenomenon usually occurs around Christmas.

A system in reverse

The normal circulation pattern in the Pacific region is produced by an area of high atmospheric pressure over the eastern Pacific, where the California and Peru Currents merge into the Equatorial Current, and a low-pressure area over the western side, above Indonesia and northern Australia. The high-pressure area causes a strong surface airflow towards the low-pressure area in the west, and this combines with the trade winds to drive the South Equatorial Current westwards, piling up warm water around Indonesia. Some of this water flows back eastwards as the Equatorial Counter-Current. In normal years, sea levels around Indonesia are about 50cm higher than they are off Ecuador.
Every few years, the pressure difference between the east and west Pacific regions breaks down temporarily and the trade winds in the central and western Pacific relax. This event, known as the Southern Oscillation, produces a sudden reversal in wind and water flow. The warm water that is being pressed up against Indonesia surges back across the Pacific as a greatly enlarged Equatorial Counter-Current. This gigantic body of warm water has a surface area one and a half times the size of the USA and contains a vast reserve of heat. It would require the energy from 1 million 10-megaton hydrogen bombs (a 10-megaton bomb is roughly equivalent to 770 Hiroshima bombs) to heat an equivalent amount of seawater to the same degree. Being less dense than normal seawater, the warm water floats rather like an iceberg, mostly submerged but protruding up to 1.5m above the surface of the surrounding ocean.