NZ just had its warmest winter - again

Along with warmer temperatures, winter also brought a series of major storms and floods: notably a mid-July deluge that forced the evacuation of half of Westport. Photo / George Heard

New Zealand just had its warmest-ever winter - a record that might be have been more notable had the previous one not been set just last year.

Meteorologists are pointing to a handful of local and regional drivers for the season's overall temperature average coming in at 1.32C above normal - but also the now-clear influence of global climate change.

The figures, derived from the Niwa's "seven-station" temperature series reaching to 1909, also show seven of the 10 warmest winters on record have occurred since 2000.

Last year's winter came in at 1.14C above average.

Niwa meteorologist Nava Fedaeff also said 76 locations across the country that experienced a record or near-record warm winter.

To put this winter's record warmth in perspective, she delved into historic weather records and found that the last time New Zealand experienced a similar sequence of events was 50 years ago.

The winter of 1970 was at the time New Zealand's warmest winter on record only to be beaten by the winter of 1971.

"What was considered to be unusually warm at the time is no longer considered unusual," she said.

"The winter of 1971 now stands in 13th place of the temperature rankings while the winter of 1970 is 18th."

What may have been considered record-breaking in 1970, was also now considered near average.

"For instance, the once record-breaking winter 1971 is 0.75C cooler than the winter we have just experienced."

A key difference could be found in carbon dioxide concentrations measured by Niwa at Baring Head, near Wellington.

In the early 1970s, they were 320 parts per million - today they are 412 parts per million.

So what did the two periods have in common?

The years 1970 and 1971 were both La Niña years featuring warmer than usual coastal sea temperatures and higher than normal pressure over and to the east of the country, which led to more northerly and northeasterly winds than normal.

The winters of 2020 and 2021 were also influenced by La Nina, warm coastal waters, frequent high pressure and more northerly and northeasterly winds than normal.

"These similar winters, decades apart, show us that there are key natural ingredients to getting a warm winter," Fedaeff said.

"But adding climate change to the mix is like taking the same recipe and swapping plain flour for self-raising."

Along with warmer temperatures, the season also brought a series of major storms and floods: notably a mid-July deluge that forced the evacuation of half of Westport, and the disastrous storm that struck West Auckland last weekend.