Will it, won't it? El Nino has us guessing

More drought for the country's east coast is possible if the increasingly likely El Nino event lands this year.
George Empson

More drought for the country's east coast is possible if the increasingly likely El Nino event lands this year.

After fizzled expectations last year, meteorologists have declared a high likelihood of this year being an El Niño event.

No-one knows exactly what it will bring, or how strong it will be, but everyone seems to agree the chances of it occurring are high, about 70 per cent.

How do they know and what does it mean for us?

El Niño along with its sister-pattern La Niña are opposite extremes of the El Niño Southern Oscillation (ENSO) cycle.

When neither are present, westerly trade winds across the Pacific pile warm water near Indonesia and cool, nutrient-rich waters well up along the coast of South America.

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Around Peru, that cool water supports the anchovy fisheries but locals noticed that in some years, around Christmas, warm water would stream in and the anchovy stocks would fail.

They named the effect El Niño - the little boy, or the Christ child - although we now know the effects span much farther than the Peruvian coast.

An El Niño arrives on a cycle of about every three to seven years. There are several clues that one is brewing, all of which meteorologists have been keeping a keen eye on for over a year.

The Southern Oscillation, first documented in the 1930s, is clearest when comparing surface air pressure at two sites: Darwin, Australia and Tahiti.

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Higher-than-normal pressure at one site is closely tied to lower pressure at the other, and vice versa. The pattern see-saws back-and-forth every few years and is measured via the Southern Oscillation Index (SOI): the difference in pressure between Tahiti and Darwin.

Very low values (i.e. lower pressure at Tahiti, higher pressure at Darwin) indicate El Niño conditions, while high values correspond to La Niña.

The SOI has been negative since July 2014, which had meteorologists thinking El Niño would have arrived last year, but it didn't quite materialise.

During an El Niño, the trade winds slacken off or even reverse. It's another good clue that El Niño is on its way, but that never happened last year.

But since January the trade winds have been weakening and they got a bigger kick in March.

Cyclone Pam, which struck Vanuatu, appeared to have "kick-started" El Niño, MetService meteorologist Georgina Griffiths says.

"Cyclone Pam produced one of the strongest reversals of the tropical trade winds seen in recent years. Known as 'westerly wind bursts', these can kick-start El Niño, since they allow warmer waters to push towards South America." 

That set waters in the eastern equatorial Pacific warming to the point that temperatures crossed El Niño thresholds: clue number three.

A tongue of warm water - more than 2 degrees Celsius above normal - had developed across the equator, from South America to the Solomon Islands.

El Niño is driven by positive feedback - changes to water temperature drive changes in the atmosphere, which in turn amplify the changes in the ocean below. 

That means if we are set for an El Niño, it will last until early next year when the natural cycle of sea surface temperature winds down.

What does it mean for New Zealand? Over winter, the country is expected to feel the bite of more southerly and southwest winds, bringing cooler temperatures.

But with prevailing winds from the west over summer, the western parts will see more rain coming off the Tasman with parts of the east coast potentially set to dry out.

Across the ditch, parts of Australia are already in drought - or have not recovered from the last drought - which means a dry El Niño spring could worsen the situation.

ENSO events account for some 25 per cent of New Zealand's yearly variation in rainfall and temperature, so there are no guarantees what will or won't happen this year. Not every El Niño means drought for the east, just as drought can also occur in non-El Niño years.

But meteorologists and farmers will be keeping a close eye on how this pattern develops.

 - Stuff

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