Climate scientists have given a 50/50 chance of New Zealand enjoying a typical summer – or one even balmier than normal.

Niwa's summer outlook, issued today, also forecast warmer seas and gave a near-certain probability of an "unconventional" and long-lingering El Nino climate system swinging into place over coming months.

Between now and the end of February, mean temperatures across the country were about equally likely to be near or above average, while rainfall levels would be near normal for most places.

That was with the exception of the north of North Island, which was just as likely to get below normal rainfall, and the west of the South Island, which had an equal chance of getting a higher than normal level.

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Amid higher pressure in the atmosphere from the Chatham Islands across to the Australian Bight, New Zealand could expect a combination of westerly and mixed wind flows over the season.

On the ocean, above or near average sea surface temperatures were expected.

Coastal waters were already warmer than normal – especially around the northern South Island, where sea surface temperatures were now more than 1C higher than usual.

Source / Niwa
Source / Niwa

Yet, because the ocean might not fully "couple" with the atmosphere in the next few months, scientists warned of potentially heightened variability – meaning the season wouldn't be persistently warm like last summer.

Meanwhile, the tropical Pacific Ocean was already showing signs of weak El Nino conditions, but the climate system would only become locked in once the ocean and atmosphere had linked up and began acting in unison.

Nonetheless, international models gave a 94 per cent chance of the tropical Pacific transitioning to an El Nino in the next three months – and an 85 per cent chance of it having bedded in between March and May.

In fact, there was an unusually high probability – a 66 per cent chance – of the system hanging about through to next winter, signalling a potential two-year event.

In New Zealand, El Nino systems typically brought cooler, wetter conditions to regions that were normally wet, and often drought to areas that were usually dry.

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Source / Niwa
Source / Niwa

Farmers in the western, wetter parts of the country often faced significant damage to pastures from too much rainfall, and it was also harder for stock to thrive in the constant wet.

But Niwa reported that this system – if it did eventuate – would differ from the typical patterns.

Another index covering the eastern Pacific, near South America, continued to show significant variability – leading scientists to expect the development of an El Nino "Modoki", in which sea surface temperatures would be at their warmest there, rather than in the central Pacific.

As far as its effects on New Zealand went, the event would probably fall somewhere between moderate and weak.

Niwa has also forecast a near-normal tropical cyclone season, with seven to 11 systems forming across the southwest Pacific between now and April, and at least one former cyclone coming within 550km of New Zealand.

If one did come close to the country, it had equal probability of passing east or west of Auckland and the North Island, likely bringing significant rainfall, extreme winds, hazardous marine conditions and coastal damage.

One of the seven "analogue" years – or those with similar climactic conditions to this year - that forecasters used for building their outlook included two ex-tropical cyclones passing within 500km of Auckland, while the others had either one or none.

This season's relatively calm outlook stood in stark contrast with the three ex-tropical cyclones that hammered New Zealand in 2017-18.