Another year has gone down among the hottest on the books - continuing a warming trend that should be anything but welcome.

Just after it was revealed that 2019 was the second hottest year globally, Niwa today announced that last year was New Zealand's fourth warmest on record.

Sadly, the result was more of the same. Five of the past seven years have been among New Zealand's hottest - and it's now been 35 months since the country saw below average temperatures.

"This warming trend is something that not only New Zealand, but the whole world is seeing," Niwa meteorologist Ben Noll told the Herald.

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"Certainly, that trend is not our friend in this case."

And there was nothing to suggest that trend would change as we entered a decade that's already seen New Zealand skies turn orange from bushfire smoke.

"Winter is becoming shorter, summer is becoming longer, and things like temperature extremes, people are going to become more and more accustomed to with time."

Warm, wet and wild

New Zealand's nationwide average temperature for 2019, calculated using stations in Niwa's seven-station temperature series which began in 1909, came in at 13.37C - or 0.76C above the 1981–2010 annual average.

That wasn't far below New Zealand's hottest-ever year, 2016, which had an average of 13.45C, or the years 1998 and 2018, which remained tied at second equal.

The year began with our third warmest January, bringing widespread heatwave conditions and record-breaking temperatures in some places.

The year's highest temperature, and New Zealand's 18th-equal warmest - 38.4C - was measured at Hanmer Forest in the South Island.

A central Pacific El Niño event, which persisted through to July, brought frequent bouts of high pressure with widespread sunny and dry weather to start the year.

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By the end of February, Nelson observed a 40-day dry spell which was the fourth-longest dry spell on record there, while Tauranga and Hamilton had 36 consecutive dry days – their third-longest stretch.

That dryness drove multiple blazes in February, most notably at Pigeon Valley in Tasman, where New Zealand's biggest wildfire in 70 years ravaged 2300ha of pine forest, razed a home and forced the evacuation of a town of 2500 people.

That fire - caused by a tractor's plough in a stony, tinder-dry paddock, amid 28C heat, low humidity, and blustery, warm wind - also prompted the largest aerial firefight in New Zealand's history, involving 23 helicopters and two planes used at its peak.

Last February's Pigeon Valley fire ravaged 2300ha of pine forest, razed a home and forced the evacuation of a town of 2500 people. Photo / NZ Herald
Last February's Pigeon Valley fire ravaged 2300ha of pine forest, razed a home and forced the evacuation of a town of 2500 people. Photo / NZ Herald

A key climate driver and contributor to the year's hot start was the presence of above average sea temperatures around our coastlines.

Some coastal areas around Hawke's Bay and Canterbury experienced marine heatwave conditions for a time and marine heatwave conditions also persisted in the Tasman Sea through to March.

Those warmer seas also provided extra energy for passing storms, such as a deluge in late March that soaked the western South Island and washed out and the Waiho River bridge on State Highway 6 .

A general trend of warmth and dryness carried on through autumn and winter, which ended up as the fourth and seventh hottest on record respectively.

It was also the driest January to June on record for Auckland, Hamilton, Whangārei, Whitianga and Kaitaia.

Snow was in short supply over most of winter, and it wasn't until the start of August that the season became more active.

The cooler temperatures at the end of winter and start of spring were also influenced by a rare major Sudden Stratospheric Warming event which occurred in the polar stratosphere.

The bridge over the Waiho River at Haast was washed away amid a late March deluge. Photo / Supplied
The bridge over the Waiho River at Haast was washed away amid a late March deluge. Photo / Supplied

Despite several sharp cold snaps, temperatures as a whole were near average for the time of year in September and continued on the near average note in October, before prevailing northwesterlies in November brought unseasonably warm temperatures and New Zealand's warmest November on record.

Another key climate driver during spring 2019 was a strongly positive Indian Ocean Dipole (IOD) event in October and November.

The IOD's hallmark was cooler than average sea surface temperatures in the eastern Indian Ocean near Indonesia and warmer than average sea surface temperatures in the Arabian Sea.

This particular IOD event was of near-record strength and caused abnormally dry conditions across Indonesia and Australia during the end of 2019 - one dramatic result being the widespread bushfires now causing devastation across the Tasman.

A dairy cow rests in a browned paddock at Paengaroa, Bay of Plenty, on February 7, 2019. Photo / NZME
A dairy cow rests in a browned paddock at Paengaroa, Bay of Plenty, on February 7, 2019. Photo / NZME

For New Zealand, it brought more westerly quarter winds than normal during spring, from cooler, drier southwesterlies in October to warm, moist northwesterlies in November.

The year ended on a cool and wet note in the South Island.

During the first eight days of December, a prolonged period of northwesterly airflows over the country delivered persistent rainfall to the headwaters of the South Island lakes and rivers.

Meanwhile a dry December for the upper North Island led to rapidly depleting soil moisture levels to start the new decade.

Taste of the future

Victoria University climate scientist Professor James Renwick noted that, while there were some cold spells in 2019, these were far outweighed by high temperatures, with over 100 new daily high temperature records broken.

He added that, in the past two decades, there were only four years with annual mean temperatures below the 1981-2010 average - putting the other 80 per cent on the warmer side.

That compared with the first 20 years of the record - 1909 and 1928 - in which just four had annual mean temperatures that were above the 1981-2010 average.

"That's how a warming climate works, we see ups and downs but the chances of a warm year are increasing all the time," Renwick said.

Renwick noted that the westerly winds that blow across New Zealand proved somewhat stronger than normal across 2019, contributing to the pattern of dry conditions in the north and east of the North Island and wet conditions in the west of the South Island.

"This pattern is what we are likely to see more of as the climate changes this century, with more frequent drought - and increased fire danger- in eastern regions and in the northern North Island," he said.

Source / Niwa
Source / Niwa

"Heavy rainfall events were peppered around the country as usual, with quite a few thunderstorm events with hail and lightning.

"The average amount of moisture in the air is strongly related to temperature, so as the climate warms, heavy rainfalls become heavier and flooding becomes more common.

"The phenomenal metre of rain that fell in two days at Cropp River is something that we are likely to see more of, and see exceeded, in future years."

The best estimates of New Zealand temperatures are for an expected increase of about 1C by 2040, and 2C by 2090.

However, owing to the different emission scenarios and model climate sensitivities, the projections of future warming cover a wide range, spanning from 0.2C to 2C by 2040 and 0.7C to 5.1C by 2090.