Explainer: what are El Nino and La Nina?

El Nino has a large impact on Australia’s yearly climate, and people often talk about less rain and hotter conditions because of it. But what is El Nino?

What is El Nino?
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El Nino is a change in the Pacific Ocean’s temperature. It is one phase in a system known as Southern Oscillation.

The system swings from El Nino to La Nina conditions, with normal spells in between, every four years.

Normally, trade winds blow from the eastern Pacific towards the west. This causes warm water to accumulate in the west.

Warm sea surface temperatures in the western Pacific cause hot air to rise and clouds to form, in a process called convection. The air cools and descends in the eastern Pacific, where it is blown westward again.

El Nino events mean a reversal in these conditions, with cool water in the western pacific and warm water in the east.

While trade winds normally blow east to west, during El Nino events the winds can stop and even turn back towards the Americas.

 

What does El Nino mean for Australia?
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Rainfall is usually reduced in eastern Australia during El Nino events. Observations from the Bureau of Statistics (BOM) show this effect.

“Nine of the 10 driest winter–spring periods on record for eastern Australia occurred during El Niño years,” the Bureau of Meteorology says.

These El Nino events are often associated with widespread droughts and they commonly mean warmer temperatures in Australia, particularly in southern areas.

The effects of El Nino are typically greatest in the winter and spring periods, with some common effects shown on this map.

In Australia’s north, monsoons occur later in El Nino years than in La Nina years.

El Nino events tend to begin in autumn and generally last about a year, the BOM said, but the events can last much longer. 

However, these hotter and dryer conditions do not mean Australia is safe from storms, like the wet weather and flooding in northern NSW in early 2016.

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What is La Nina
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La Nina is almost the direct opposite of El Nino, the BOM says.

"It's a bit like the nuetral phase has gone into overdrive," the BOM says in one of its videos.

La Nina means warmer than average sea surface temperatures in the western Pacific.

Stronger trade winds mean the patch of cool air across the Eastern and Central Pacific Ocean is larger, with more warm water accumulating in the western Pacific.

La Nina events also generally start in Autumn and last about a year, the BOM said. 

However, the BOM also said no two El Nino and La Nina events were the same.

What is the La Nina’s effect on Australia?
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The effects on Australia during La Nina events include more rain and cooler temperatures in parts of the country. These effects peak in winter and spring.

The effects of La Nina and El Nino are different at different times of the year, and in different places, these BOM maps show.

Tasmania, Victoria and South Australia receive above-average rainfall during La Nina years, the BOM says.

Australia was more prone to cyclones during La Nina events and no part of the country had below-average rainfall in La Nina years, the BOM said.