It might be hot – and abnormally so, as meteorologists keep telling us – but this month still doesn't stack up to last January's record warmth.

January 2018 finished up with a mean temperature of 20.2C, or about 3C above average, and went down as the hottest month that New Zealand had experienced in 150 years of observations.

By contrast, this month had been running at about 1.5C above average, climate scientist Professor Jim Salinger said.

But he noted this week's heatwave had still managed to break some records of its own.

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For about 10 minutes yesterday afternoon, the temperature in the Wellington suburb of Kelburn rose to 30.3C – the highest since records began in 1927.

Auckland also hit 30C, the first time it has passed that milestone since the same time last year.

The warmest place was Whenuapai, which reached 30.3C.

Graphic / NZHerald
Graphic / NZHerald

Despite the current heat, Salinger thought it unlikely that the record-hot summer of 2017/18 would be topped by this summer, which was so far tracking at nearly 1.4C above average.

"We would need an incredibly warm February to beat it - somewhere in the order of departures of 3C to 3.5C above the 1981-2010 normal."

A just-published review of last summer, led by Salinger, described the extraordinary mix of factors involved.

They included a persistently positive Southern Annular Mode which brought fewer cold fronts, a La Nina climate system, and background global warming.

The sweltering heat of January 2018 sent Ross Maxwell, 21, into the water at Tauranga's waterfront. Photo / File
The sweltering heat of January 2018 sent Ross Maxwell, 21, into the water at Tauranga's waterfront. Photo / File

The unmistakably warm season brought hellishly humid nights, localised droughts, a boom in insect numbers and the damage and deluges of cyclones Fehi and Gita.

It also caused the biggest-ever melt on the Southern Alps, which lost nearly 10 per cent of their ice over the season, along with major disruption of marine ecosystems and very early maturation of wine grapes.

Source / Niwa
Source / Niwa

Salinger and his colleagues said the season, which broke an 84-year-old record for warmth, offered a good analogue for possible mean conditions in the late 21st century.

He noted average temperatures had already risen by some 0.8C in the past four decades, with temperatures now about 1C warmer than they were a century ago.

With further global warming, what we are seeing now and last year's summer could be the norm in the years 2081-2100.

Last summer's warmth also coincided with a freak marine heatwave which pushed sea surface temperatures to between 2.5C and 4C above average through much of December 2017.

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

The many southern anticyclones had calmed the waters in the Tasman Sea and east of New Zealand, allowing heating of the upper layers of the ocean.

Niwa's monitoring of the past fortnight showed ocean temperatures were again sitting above average, but not to the degree of last summer.

Sea surface temperatures were currently 1.3C above average for the time of year, with some patches of sea in central and western parts of the New Zealand region having warmed to 2C to 3C above average.

Source / Niwa
Source / Niwa

Coastal waters were generally 0.5C to 1.5C above average – a drop from two weeks ago – but marine heatwave conditions were still likely occurring in parts of the central and western Tasman Sea and in coastal waters east of the North Island.

Compared with the same two-week period in 2018, New Zealand coastal waters were 1C to 3C cooler, although the western Tasman Sea was still 1C to 3C warmer.

Source / Niwa
Source / Niwa

The hot air mass parked above New Zealand this week had also pushed up sea surface temperatures by 1C to 2C in just the last several days, Niwa reported.

Source / Niwa
Source / Niwa

A southerly change was forecast to occur from Friday into Saturday, ending the spell of unusual warmth.

Heading through the first half of February, weather patterns were forecast to have more variability than what was observed during January.

And while sea surface temperatures were expected to remain above average for the time of year, marine heatwave conditions were unlikely to develop in new areas.

Graphic / NZHerald
Graphic / NZHerald