El Niño Southern Oscillation• After the seasons, El Niño and La Niña are the single largest causes of year-to-year climate variability on Earth.

• In the past two decades, the El Niño Southern Oscillation (ENSO) has become one of the most studied phenomena on the planet, because it is also one of the most feared.

• An El Niño event is the reversal of the usual weather patterns that occur in the tropical Pacific, caused by irregular fluctuations in sea surface temperatures. The oceans have a considerable heat storage capacity which makes them a major influence on world climates. If ocean temperatures change, this will have a considerable effect upon weather patterns in adjacent land masses.

• The result is a thick layer of warm water over the western Pacific. In the east, the removal of warm surface water (blown westward) allows a mild countercurrent of cool, deeper water to rise. Cool upwelling water near Peru creates high pressures off the coast of Peru, helping reinforce the trade wind circulation.

Thermocline = boundary between cold, deep water and warmer upper layer

Indonesia and Australia (west)

South America (east)

West East

The westward flow of water raises the sea level in the western Pacific some 20-40cm higher than it is next to the South American coast.

Walker Circulation

Pacific-Rim Conditions: An El Niño Year• An El Niño event is different. All it takes to trigger is a slight

relaxation of the trade winds, when a feedback loop sets in. The weaker winds mean the mass of warm water in the west suddenly surges eastwards, forming a warm layer of water much closer to South America than usual.

• That warmth in turn warms the air above, which lowers the pressure of the atmosphere, and so the winds weaken even more. So more water surges eastwards and suddenly there is a reversal. The whole of the Pacific is effectively covered with an extra layer of warmth and the winds change direction, settling into a new pattern in the opposite direction.

South America (east)

Indonesia and Australia (west)

But with El Niño the nutrient-rich, cooler waters the fish need are held too deep by the layer of warm water above, so the surface sea becomes hostile and the fish flee south or die.

Normally the cooler water rising off the coast, waiting to be blown west on the trade winds, brings rich nutrients from the deeper ocean and supports some of the world’s most abundant fishery areas.

The close link between El Niño and the Southern Oscillation means meteorologists often refer to the El Niño Southern Oscillation (ENSO) to describe the whole phenomenon. Generally El Niño refers to the oceanic properties and Southern Oscillation refers to the atmospheric component.

Summary: In an El Niño event, the winds change direction as part of a wider periodic shift of atmospheric pressure patterns over the Indian and Pacific oceans, which has been known as the Southern Oscillation since the 1920s.

*Remember, 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.

April

May

June

September

The 1997 El Niño developing and moving eastwards across

the Pacific.