Niwa weather outlook: What an ‘atypical’ El Niño means for your summer

An “atypical” El Niño summer is set to bring plenty of variability amid the big climate driver’s traditional wet-and-dry flavours for New Zealand’s west and east – starting with a bout of humidity for the North Island next week.

For Kiwi summers, El Niño usually means hot, dry conditions for the northeast, wetter weather for the southwest, and westerly winds over most of the country.

Only, as Niwa’s just-released outlook for the next three months shows, things aren’t likely to run entirely to script this season.

While Kiwis will still feel the effects of a classic El Niño pressure set-up - driving more and stronger westerly and northwesterly quarter winds on to New Zealand - Niwa expected higher variability in circulation patterns and air flows compared with past events.

That variability – partly due to an equatorial West Pacific still running unusually warm – raised the potential for the odd big subtropical rainmaker to find its way down here, albeit not with the relentless frequency of last summer.

“We’re expecting atypical impacts from an atypical El Niño, with excessive warmth not just in the Pacific but right across the planet,” Niwa forecaster Ben Noll said.

“As it pertains to our weather, we will feel those unusual impacts right out of the gate, with the first week of the month looking pretty sodden across the country.”

Niwa picked rainfall levels to be near normal in the north of the South Island, near or above normal totals in the west of the South Island, and near or below normal levels across the remainder of the country.

Further south, strong lows were expected to deliver heavy rainfall for the western and lower South Island in mid-to-late December – raising the risk of flooding over the Christmas period.

Temperatures, meanwhile, were most likely to sit above average in the east of both islands and the north of the North Island - and near average or above average in all other regions.

For those warmer, drier areas, Noll said summer temperatures reaching the mid to high 30s remained “well within the realm of possibility”.

“Given the northwesterly flows that are favoured, either Australian air masses or subtropical air masses can produce that level of warmth.”

While there was still a risk of summer dry spells for several eastern regions, Noll said this El Niño didn’t appear to be posing the threat of months-long drought that farmers endured in horror events like 1997-98.

“We might have dryness for three, four or five weeks – but then that might be followed up by a different flavour of weather that could last a week or two, and almost take the edge off.”

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.