Kiwis can expect a summer of two halves – one that's dry in the south, wet in the north, but generally warm everywhere – if an anticipated La Niña climate system brings its usual flavour to the table.
Meteorologists have also pointed to the potential for warm sea temperatures – and possibly more tropical cyclone activity.
A week after Kiwi climate scientist Professor Jim Salinger told the Herald a La Niña system had now developed in the Pacific Ocean, Australia's Bureau of Meteorology yesterday issued an official declaration.
Niwa is holding off doing the same – but could likely move from its current watch status of La Niña "alert" to also confirming an event over coming weeks.
Salinger said he expected the coming La Niña to be a moderate-level event – or similar in scale to one that played out in 2011-12.
He predicted it to peak around December and January, with a switch from northwest to northeast prevailing winds around the close of spring.
In the meantime, Niwa's just-issued seasonal outlook has predicted above-average temperatures for the rest of 2020, along with an elevated chance of high temperature extremes - particularly on days with a strong northwest wind.
Air pressure, too, was forecast to be higher than normal to the southeast and lower than normal to the north of New Zealand.
That was expected to be associated with developing La Niña-like northeasterly air flow anomalies, although a westerly flow anomaly, which could be strong at times, was favoured to continue for much of October.
So what sort of weather might we get further out as the La Niña properly beds in?
A summer divided
In a La Niña event, ocean water spread from the coast of South America to the central tropical Pacific cools to below average.
This results from stronger than normal easterly trade winds, which churn cooler, deeper sea water up to the ocean's surface.
While water in the eastern Pacific runs unusually cool - suppressing cloud, rain and thunderstorms – sea temperatures in the far west of the ocean warm to above average temperatures.
Here in New Zealand, more northeasterly winds arrive, bringing rainy weather to the North Island's northeast, but drier conditions to the south and southeast of the South Island – or the reverse of an El Nino.
Niwa meteorologist Ben Noll said the northwest-to-northeast change may spell a wet summer season for exposed regions like Northland, the Coromandel and the East Cape.
"At the same time, areas like Taranaki, Manawatu, and down through much of the South Island – especially interior or western parts – are sheltered [from] those very same northeasterly winds," he said.
"As that northeasterly wind travels across the North Island then onto the South Island, the air is drying out as it comes south.
"And La Niña comes often comes with big areas of high pressure that sit near the South Island - and sometimes right over the South Island."
It was likely we'd start to see signs of this general pattern late in October, and then across November and December, he said.
"Sometimes, those big areas of high pressure can also kind of cradle the low pressure as it comes down from the north – and it can become stuck near the upper part of the North Island," he said.
"Historically, that's when we can have a lot of rain in a short period.
"We're still probably a month or two away from that becoming more of a risk, and it doesn't look like it's on the cards right now.
"But, as we go through the upcoming three-month period, it's a gradual thing that will build in our climate."
Aucklanders, especially, may also be in for more humid weather over summer, as northerly flows draw warm air and moisture down from the tropics.
Noll said many would recall Auckland's excessive mugginess during the record-hot summer of 2018, which came after a weak La Niña system formed over the preceding months.
That summer notably also saw a freak "marine heatwave", when sea surface temperatures were pushed to 1.5C above average, and as high as 6C above normal in some spots off the West Coast.
It was also the result of La Niña-skewed conditions, along with blocking anti-cyclones centred over the Tasman Sea, fewer low pressure systems and a strongly positive Southern Annular Mode that calmed ocean waters – all against the backdrop of climate change.
At their most severe, marine heatwaves can have dramatic effects on land, pushing up air temperatures, melting glaciers, and putting growing seasons weeks ahead of schedule.
The big 2017-18 event also saw cascading losses of mussel beds and other seaweeds along the coasts of Southland and Otago, while beaches across New Zealand became crowded weeks earlier than usual.
Meteorologists have been tipping more warm seas this summer – and coastal waters around the country have already been unusually warm over recent months.
Noll said historic records drew a clear link between La Niña and above-average temperatures in the Tasman Sea.
"And then there's what we call analogue years – or years in the past that have climatic similarities with the present," he said.
"Many of those year's springs and summers have come with increased ocean temperatures, especially in the Tasman Sea."
Weather systems that have swept in over the past few weeks had churned up seas and brought about a moderate drop in coastal water temperatures – but they were still sitting "a touch" above average.
"But as we go through the upcoming three months, high pressure is expected to become more and more of a player in our weather pattern.
"So that means more sunshine, less wind on and northeasterly air flows, which can help bring down some warmer air from the subtropics and tropics to our north – and this would help to warm the ocean temperatures here in the Tasman Sea and around New Zealand."
Meteorologists consider a "severe tropical cyclone" one that blasts as hard as 118km/h, and each November-to-April season, about 10 of them form in the Southwest Pacific basin.
Only a few of those ever reach category 4 strength, where mean wind speeds are more than 159 km/h, or higher.
Vanuatu and New Caledonia typically experience the greatest activity, with an average of two or three named cyclones passing close to land each year.
At least one comes within 550km of New Zealand each season - usually around February and March.
Niwa is still a few weeks away from issuing its tropical cyclone outlook for 2020/21, but Noll said La Niña could also have an influence on the picture.
"What can happen is the Southwest Pacific Convergence Zone, which is the area of converging winds that sits near the Pacific islands to our north, can be more active than normal during La Niña events.
"Ocean temperatures from Fiji to Vanuatu to New Caledonia out into the Coral Sea can also be warmer than average.
"It's one of the reasons why Queensland often sees an elevated risk for ex-tropical cyclones during La Niña events.
"And those systems that form to our north - depending on the pressure pattern that is sitting further south - will dictate basically how likely they are to track southward.
"So, if there are more cyclones in that part of the world, there's a better chance overall that New Zealand will feel the impacts of one of those systems at some point during the cyclone season."
Noll pointed out that the 2017-2018 season saw three cyclones - Gita, Hola and Fehi – and Gita's damage alone led to more 4,000 insurance claims, costing insurers just over $28 million.
"So, while we average one a year, La Niña kind of tips the scales a little bit toward that higher end of the scale."