The same ingredients that fuelled the most intense marine heatwave ever observed in New Zealand waters have combined once again, raising the odds of abnormally warm seas and scorching days this summer.
Niwa's latest seasonal outlook has put the chances of another La Niña climate system forming over the next three months at 80 per cent, and also flags the potential for warmer-than-average sea surface temperatures around the country.
With summer still a month away, sea temperatures have already been running at between 0.5C and 1C above average, Niwa forecaster Ben Noll said.
"At the end of November, if we're at a point where ocean temperatures have reached past 1C to 1.5C above average, we'll be watching out for things like marine heatwaves," he said.
"That may lock in plenty more warmth on land as we go through December and January, because we know that the highest correlation between air and ocean temperatures tends to be over the summer."
While balmier seas might be good for beachgoers, marine heatwaves have also proven to have severe impacts on ocean ecosystems and our industries that rely on them.
An unprecedented event in 2017-18 formed the backdrop to New Zealand's hottest-ever summer - and came with dramatic consequences.
Air temperatures over the country reached between 1.7C and 2.1C above average, while sea surfaces heated up to between 1.2C and 1.9C above average.
Glaciers melted as some pockets of ocean off the South Island West Coast warmed to 6C above average, while elsewhere, mussel beds suffered cascading losses and vineyards saw early harvests.
That heatwave – which was followed by another the following summer – was created by a mix of ocean and atmospheric drivers.
They included more blocking anti-cyclones centred over the Tasman Sea, fewer low-pressure systems, and a strongly positive Southern Annular Mode (SAM).
When this effect was positive, the massive storms that rage in the "roaring forties" and "furious fifties" latitudes in the southern oceans contract toward Antarctica, instead of in our direction.
The combination of that and a strong La Niña in the tropical Pacific - known to drive more anticyclones east of New Zealand – brought many more anticyclones to our region, while blocking cold swells surging up from the deep south.
Scientists also singled out the hand of human-driven climate change, noting that the Tasman had been warmer over the years leading up to the events.
Climate scientist Professor Jim Salinger said the stage was set for another event, given the forming-up La Niña, warmer seas and a strongly-positive SAM.
As well, he added, a separate climate indicator called the tripolar index was in a negative phase, which encouraged La Niñas and sea surface temperatures around New Zealand to be above average.
"So, all of the ingredients in the atmospheric stew are right for a heatwave."
Noll also pointed to some striking similarities with the prelude to the freak heatwave that engulfed the country four years ago.
"It's early, but one of the interesting things is how similar things are looking right now to what came before that summer of 2017-18," he said, adding that heated oceans culminated in January 2018 going down as the warmest month ever recorded in New Zealand.
"So, the same puzzle pieces are all there laid out on the table, it'll just be a matter of whether they end up fitting together in the very same way."
Noll emphasised that 2017-18 was an "extraordinary summer", in terms of the unusual combination of factors that stoked it - the marine heatwave itself was described by scientists as an unlikely event, even by mid-range estimates for 2050.
As the planet warms, marine heatwaves are expected to grow stronger, longer and more frequent.
Elsewhere in its three-month outlook, Niwa said temperatures were "very likely" to be above average across the country, with a period of particularly warm conditions from around the second week of November.
Higher than normal air pressure was predicted over the South Island and to the south and east of the country, causing more easterly winds and increasing the chance for dry spells in the South Island, and western North Island in particular.
La Niña conditions traditionally brought warmth everywhere, but, unlike El Niño, drove more rain to the northeast and drier conditions to the south and southeast of the South Island.
That wasn't however the trend that played out over last summer's La Niña.
Over the November to January period, rainfall was also most likely to be below normal in the west of the South Island, near normal in the north and east of the North Island and about equally likely to be near normal or below normal in all other regions.
"Occasional sub-tropical low-pressure systems can bring heavy rainfall and possible flooding to New Zealand, particularly in the northern and eastern North Island."