A "trifecta" of forces combined to make this summer what will be New Zealand's hottest ever - and experts say the influence of climate change now can't be ignored.

The unmistakably warm season brought hellishly humid nights, a top-heated Tasman Sea, the hottest single month on record, localised droughts, a boom in insect numbers and the damage and deluges of cyclones Fehi and Gita.

"New Zealand scored the trifecta this summer, with respect to heat," MetService meteorologist Georgina Griffiths said.

That triple-hit combo included a marine heatwave, a La Nina climate system in the tropics, and an extreme positive phase of weather driver called a Southern Annular Mode, or SAM.

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Climate scientist Dr Jim Salinger said the national temperature record for summer was tracking at 18.9C - 2.3C above average, and 0.4C warmer than New Zealand's previous hottest season in history, experienced in 1934-35.

Both of those summers had similar ingredients: unprecedented warmth, a very wet February, and a marine heatwave.

But this time, scientists say, climate change likely played a hidden role.

The scene had been set by intense high pressure that prevailed across the Tasman and New Zealand during November and the first half of December, bringing an unusually long period of light winds over the sea and around our coastline.

That lack of wind, and subsequent lack of waves, meant that surface ocean heat was not being churned up – and instead made conditions ripe for a build-up.

Temperature anomalies over the month to date. Image / Niwa
Temperature anomalies over the month to date. Image / Niwa

Sea surface temperatures steadily rose, to the point they became between 2.5C and 4C above average through much of December.

Incredibly, some localised spots off the West Coast even reached between 4C and 6C above normal.

People cool off at Maraetotara Falls, near Havelock North, on Christmas Day. Photo / Duncan Brown
People cool off at Maraetotara Falls, near Havelock North, on Christmas Day. Photo / Duncan Brown

Niwa meteorologist Ben Noll said the persisting phenomenon had surprised scientists by peaking three times: in early December, late January and mid-February.

"This thing has been incredible – and looking at the long-range maps, as we head into March, it seems like it's going to warm up again dramatically here in New Zealand."

While this made ideal conditions for swimming – lifeguards reported packed beaches, and much earlier in the season than usual – the impact of heatwave on sea life was still unclear.

This graphic shows the dramatic anomalies in sea surface temperature around New Zealand this summer. Image / Supplied
This graphic shows the dramatic anomalies in sea surface temperature around New Zealand this summer. Image / Supplied

Some dramatic observations have included snapper being caught in Doubtful Sound for the first time ever, an early-season bluebottle jellyfish boom and increased sightings of stingrays in Otago Harbour.

On land, hot temperatures in the South Island had caused a massive melt-off, with a marked loss of permanent snow and ice in the Southern Alps.

In most-wine growing regions, the hottest average grape flowering temperatures recorded in more than 20 years had put the season several weeks ahead of normal.

Salinger said it was the SAM that had really cranked up the heat this summer.

The SAM measured the north–south movement of the westerly wind belt that circled Antarctica, dominating the middle to higher latitudes of the southern hemisphere.

Ex-Cyclone Gita damage in Riwaka, Nelson, New Zealand: A hug for Val A'Court from friend Monika Walter as Val starts the clean-up of her silt-covered property. Photo / Tim Cuff
Ex-Cyclone Gita damage in Riwaka, Nelson, New Zealand: A hug for Val A'Court from friend Monika Walter as Val starts the clean-up of her silt-covered property. Photo / Tim Cuff

It was the changing position of that westerly wind belt which influenced the strength and position of cold fronts and mid-latitude storm systems over New Zealand.

A positive SAM had meant there were weaker westerly winds than normal over the South Island with higher pressures - and less cold fronts crossing New Zealand with their "commando raids of cold air", Salinger said.

A graph showing how the Southern Annular Mode - indicating storminess in the Southern Ocean - has been dominated by a positive phase over summer. Image / MetService
A graph showing how the Southern Annular Mode - indicating storminess in the Southern Ocean - has been dominated by a positive phase over summer. Image / MetService

"In its positive phase, it encourages blocking highs to the east of the country, and sometimes bulging back over New Zealand, with mild northerly airflows across New Zealand," Griffiths added.

"Basically, positive SAM conditions support above-average temperatures for many regions of the country."

The third big factor, a strong La Nina, had also brought what the ocean-driven system typically does to New Zealand: higher temperatures, more east to north-east winds, and extreme humidity that's made conditions unbearably sticky in the north over the past few weeks.

The La Nina had caused the tropics north of New Zealand to "boil with convection", Salinger said, which ultimately delivered us a damp, muggy February.

The deluges from cyclones Fehi and Gita, too, were a result of very warm air passing over very warm seas – something he said was all a consequence of a warmer world.

Climate scientists had also singled out climate change as being a big driver of an increasingly positive SAM.

"More impacts will come to light as data comes in, but the hot summer is a good example of what will common later this century in the lifetime of today's children in New Zealand."

Noll expected that many of this summer's extreme features bore the fingerprints of a changing climate.

"If you are having your warmest month on record, in what is likely to be the warmest summer on record, that takes some pretty remarkable things in the climate space to occur," he said.

"I'm sure that for a lot of people, this may be the warmest summer they've experienced in their lives."