New Zealand's official climate agency has just declared autumn the fourth warmest on record.

But by one climate scientist's model, which combined sea temperatures as well as those on land, the autumn of 2019 took the top spot as the hottest on the books.

Professor Jim Salinger made that calculation, suggesting the warmest autumn in more than 150 years of records, partly based on a temperature series he assembled featuring 11 stations in the North Island and another 11 in the South Island.

This series put the season as the second equal warmest, tied at 1.2C above average with 1999, but behind autumn 2016, which came in 1.5C above average.

Advertisement

However, when recordings from the seas around New Zealand were factored in to make a wider model, the March to May period came in at 1.2C above average, or the warmest ever, Salinger said.

He clarified that these would differ from Niwa's figures, as the agency used a seven-station series to compile its summaries.

Salinger said the balmier conditions were partly due to the Southern Annular Mode - indicating storminess in the Southern Ocean – which had squeezed southern westerlies squeezed further south.

This had resulted in fewer outbreaks of cold air, and New Zealand, especially the South Island, being surrounded by oceans that were much warmer than normal.

Salinger pointed to the background influence of climate change.

"As greenhouse gases increase in the atmosphere, and global warming progresses, this is likely to be a much more frequent occurrence in years to come."

Niwa also cited warmer sea temperatures – a continuation of a marine heatwave the country experienced in summer – as having an influence.

The agency said autumn had been characterised by higher than normal mean pressure over and to the east of the country, resulting in more northerly winds than normal across the country.

Advertisement

Throughout the season, temperatures – coming in at a nationwide average of 14.4C – were mostly above average or well above average.

The warmest autumn temperature of 32.4C was recorded in Waipara West on March 5, while the lowest, a chilly minus 6C, was observed at Ranfurly on April 7.

Rainfall over much of the North Island was below normal, with isolated pockets of well below normal.

"Conversely, a large portion of the South Island received above or well above normal rainfall," Niwa reported.

Source / NIWA
Source / NIWA

Last month also proved to be the third warmest May on record, with temperatures above average or well above average across the entire country, and most unusually warm temperatures being felt in the South Island.

Of the six main centres last month, Auckland was the warmest, Christchurch was the coldest and wettest, Tauranga was the driest, Wellington the sunniest, and Dunedin the least sunny.

Over the next three months, Niwa was forecasting above or near average temperatures for most of New Zealand.