Aucklanders and others in northern regions may be in for an uncomfortably sticky summer, as Niwa confirms the arrival of La Niña climate system – and abnormal warmth with it.
Niwa's just-issued summer outlook picked temperatures to be above average across the entire country, with warm nights and "extended periods" of high humidity likely.
While air pressure was likely to be higher than normal in the south and east of the South Island, low pressure could develop north of the country – particularly later in the season.
That was expected to be linked with more easterly winds, which in turn raised the odds of dry spells about the west of both islands.
Rainfall, meanwhile, was most likely to be near normal in the north and east of the North Island – and about equally likely to be near or below normal in all other regions.'
"Occasional sub-tropical low pressure systems could bring heavy rainfall and possible flooding to New Zealand, particularly in the northern and eastern North Island," the summary added.
Niwa forecaster Ben Noll said summer's weather would be shaped by several variables in what he called a "climate driver burger".
"The patty in this burger is a La Niña - and we'll call the marine heatwave we now have under way our tomatoes."
Niwa said La Niña had now developed in the equatorial Pacific – something the World Meteorological Organisation also confirmed overnight.
Unlike its counterpart El Niño, La Niña tended to drive more rain to the northeast and drier conditions to the south and southeast of the South Island – although this classic set-up wasn't what played out during this year's La Niña summer.
In any case, the system wasn't likely to be as strong as other events observed over recent decades.
The emergence of another marine heatwave - which has daily sea surface temperatures in large swathes of our coastal waters to more than 3C above average over the past week – could also drive more warmth across the country over the season.
Conditions that have been observed over the past month have even been compared to those that foreshadowed New Zealand's record-hot summer of 2017-18.
"If this marine heatwave does persist over summer, that could invigorate weather systems that visit New Zealand from the tropics, by adding moisture, or helping retain their energy and structure."
Niwa has already warned of a slightly elevated risk from ex-tropical cyclones over the next six months, with the potential for one to two of these destructive systems swinging our way.
Each season - usually around late summer - at least one ex-tropical cyclone veers within 550km of the country, packing gale-force winds and enough moisture to drive torrential rain.
Noll singled out another ingredient in his climate hamburger.
That was the Southern Annular Mode, or SAM – a ring of climate variability that encircles the South Pole, but stretches far out to our own latitudes.
Some warm years had been put down to the SAM being locked in a mostly positive phase, which came with westerly winds farther south over the southern oceans but lighter winds and sunnier skies over New Zealand.
"There was not a single day during November where the SAM was negative – so look for that continuing to play a large part in what we get over the coming three months."
Unfortunately, he said, this combination created conditions ripe for heavy humidity – especially in northern regions.
"You might want to go out and get that fan earlier than later. The last time we were running this hot was 2017-18, when I think The Warehouse ran out of fans."
Sticky nights with high temperatures may also be noticeable as warm days, he added.
"Certainly, there are many parts of the globe where overnight maximum temperatures are increasing more quickly than daytime maximums under climate change.
"These conditions will also be pushed along by those really warm coastal sea temperatures."
The heat locked in until 2022, combined with a record-warm winter, would likely secure 2021 a place within New Zealand's five hottest years on record – continuing a dismal trend under climate change.