Extreme El Nino events likely to become twice as common in New Zealand

Tuesday, 25 July 2017

A parched Marlborough farm during an El Nino year. El Nino events can cause drought in the east of the country.

Extreme El Nino events - the sort that can bring severe droughts to the east of New Zealand and more heavy rain to the west - are likely to happen twice as often if the global mean temperature rises by 1.5 degrees Celsius.

If the temperature increases more than that, they'll happen even more often, according to a new report published in the Nature Climate Change journal. It's not good news for weather-reliant industries like agriculture.

Average annual rainfall in Rotomanu on the West Coast is 3.5 metres, but close to 6m in El Nino years. El Nino often causes increase rainfall west of the Southern Alps.

It also means that even if the 2015 Paris climate agreement - which aims to limit global warming to 2C - is kept to, more extreme El Nino events will occur.

El Nino can have a huge impact on the country's economy. For example, the drought associated with the major 1997-98 El Nino resulted in a loss of $618 million or 0.9 per cent of GDP - a figure estimated by the New Zealand Institute of Economic Research.

A woman takes a dip to escape the dry summer heat during an El Nino event.

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As well as severe droughts, El Nino can also cause stronger, more frequent westerly winds, changeable weather and colder southerlies in winter. NZ's last major El Nino event released its grip on the country in Autumn last year. That brought on a longer-than-usual summer.

Co-author of the new report, Doctor Wenju Cai, said the mean global temperature was expected to hit the 1.5C increase point between 2040 and 2050.

From then on, the number of extreme El Nino events was likely to increase from the current frequency of about four every 100 years to one per decade.

Although frequency was expected to increase, severity of El Nino events was not, Cai said.

But they could still have a 'huge impact', particularly on small South Pacific nations that had less capacity to deal with recovering from them and the tropical cyclones they were often associated with, Cai said.

Victoria University's Professor James Renwick said a big El Nino made the whole globe warmer, even Alaska and Antarctica.

'Lots of big ones [extreme El Nino events] could impact the rates at which sea ice melts.'

However, Renwick also said 'it tends to be quite hard to model El Nino events successfully.

'The group that wrote the paper are doing a pretty good job … It's plausible, but the future for El Nino is still pretty murky.'

WHAT IS EL NINO AND HOW DOES IT OCCUR?

El Nino is a climate cycle in the Pacific Ocean that has a global impact on weather patterns.

It occurs when trade winds weaken, leading to the sea surface temperature to increase in the eastern Pacific Ocean near the equator.

It's normally warmer near Indonesia, but instead it warms up near South America.

Heavy rainfall and flooding occur over Peru, and drought over Indonesia and Australia, according to NIWA.

Prediction of El Nino is strongly related to the Southern Oscillation, an atmospheric event where the surface air pressure increases in one place and correspondingly decreases in another.

It's measured by the air pressure difference between Tahiti and Darwin.

Negative values indicate El Nino and positive values are related to La Nina.

According to NIWA, La Nina causes more north–easterly winds which tend to bring moist, rainy conditions to the north–east of the North Island, and reduced rainfall to the south and south–west of the South Island.