What was the wider New Zealand region's warmest year on record proved another sign of climate change in action, a scientist says.
This month, Niwa reported 2019 was the country's fourth hottest year, with an average temperature that came in at 13.37C - or 0.76C above the 1981–2010 annual average.
That figure was calculated off Niwa's long-running seven station series, measuring land temperatures from one side of New Zealand to the other.
But by a broader measure – taking in 22 stations, and sea temperatures across New Zealand's expansive ocean estate – 2019 finished up as the warmest on the books.
That calculation by Professor Jim Salinger, an architect of the seven station series, found the region-wide average temperature was 14.3C – or 0.84C above the average.
Sea surface temperatures alone for the year were an average 14.63C – or 0.85C above average.
Salinger said the year got off to an unusually warm start, riding on the back of a marine heatwave that was the second in as many years.
Later in the year, the region saw the influence of a strongly positive Indian Ocean Dipole – a climate driver implicated in Australia's bushfire crisis – along with a warm blob of ocean sitting near the Chatham Islands.
It wasn't until spring that sea surface temperatures around the country cooled, largely because of another climate indicator called the Southern Annular Mode turning to a negative phase, and a rare stratospheric warming event above Antarctica bringing a chillier pattern.
"From about October or November onward, we've had plenty of westerlies and northwesterlies – which is why summer has been a bit hit and miss in the south of the country."
Right now, coastal waters around the country were still on the cooler side of average, Niwa meteorologist Ben Noll said.
An increase in westerly winds forecast for the coming weeks could keep them cool by driving upwelling in the sea – but hotter air temperatures also on the cards for the next seven days could warm up the sea surface.
"So it's going to be a bit of a battle between those two factors," Noll said.
"If it does get past 35C in some places next week, which is possible, that will certainly lead to warmer sea surface temperatures in areas."
Salinger felt ocean temperatures were a crucial factor to consider when making long-term temperature observations.
"Globally, the ocean heat content is at record levels – and in the New Zealand region, oceans have been running hot for some time," he said.
"When you have that heat content, all it takes is the right atmospheric conditions, and you get record temperatures.
"Or, in other words, if you get conditions that calm the oceans down – as happened last summer – then away it goes. And that extra ocean heat content is due to global warming."
Models showed that oceans were now absorbing 90 per cent of the heat added to the global climate system.
For land temperatures alone, five of the past seven years have been among New Zealand's hottest - and it's now been more than 35 months since the country saw below-average temperatures.
The warmest annual average land temperature yet recorded for the country was 13.45C in 2016; the other two hottest years were 1998 and 2018.