How does El Nino occur?

El Nino occurs as a warm water current moves eastwards across the Pacific off of the South American coast. This shifts the world's pressure and so the climate of regions changes.

Why does this warm water current move across the Pacific in the first place?

Well, no-one is exactly sure although our understanding has improved in the last few years. One of the most commonly accepted theories is that there is an east-west circulation of heat between high and low pressures. This is called the worldwide Walker Circulation. For some reason, every 2-7 years, this circulation breaks down. This means that the easterly winds decline dramatically and the warm water pulse is created around Australia.

Why does this cause unusual weather phenomena?

As the winds move over the Pacific Islands, particularly Tahiti, the conditions for tropical cyclones are now present. Additionally, because the pressure is high, the temperature of the ocean is up to 6 degrees Celsius higher than it should be. The warm water means that rainfall occurs in areas which are norm ally arid and drought elsewhere. Finally, the American jet stream is forced south, causing freak weather occurrences in Canada, the USA and Mexico. For instance, flooding and heavy snow are common in El Nino years.

What is the worst El Nino so far?

Probably the worst yet hit in 1982. It caused some $1000 million worth of damage worldwide. There were monsoons in the Pacific Ocean, fires in Borneo, dust storms in Australia, as well as worldwide droughts. Fish and bird populations plummeted all over the world.

What can be done to predict El Nino?

As yet very little. There have been some small scale programmes in Zimbabwe and Australia to predict when the Walker Circulation might break but this has proved unreliable.

What is La Nina?

La Nina is effectively the opposite of El Nino, where the jet stream moves due to a well established Walker Circulation. The last one occurred in 1989, when there were monsoons in Bangladesh, Thailand, Sudan and Australia, and severe droughts in Midwest USA.