Niwa confirms NZs warmest June, with more mild days ahead

Last month was New Zealand’s warmest June in more than a century of records – owing to more north-easterly air flows, subtropical-flavoured pressure systems and the background influence of climate change.

While it ended with a polar blast that brought snow to many spots across the country, June’s nationwide average temperature came in at a record-topping 10.6C – and the unseasonably mild trend is expected to continue for the rest of winter.

June’s final figure also happened to be 2C above the 1981-2010 average. It was just one of only 13 times since 1909 that a month’s temperature sat that high above the 30-year baseline.

A total 24 places saw their own warmest June, including Auckland, Wellington, Tauranga, Whakatane, Gisborne, Taupo, Porirua, Westport, Hokitika and Greymouth.

Hastings, which registered its second warmest June since records began in 1965, locked in the month’s highest temperature – 22C on June 26 – jointly with Leigh, where the mercury reached that high on June 19.

That was the same day that Middlemarch registered June’s lowest temperature – 7.4C.
Throughout the month, mean sea level air pressure was above normal to the east of the country – something that was linked with more northeasterly air flows than usual.

“The prevalence of these air flows, occasional low pressure systems that transported warm, humid air down from the subtropics, and ongoing background warming from climate change meant it was a very warm start to winter throughout the country,” Niwa reported.

Niwa meteorologists have also pointed out the influence of a vast swathe of warm water in the West Pacific, built up during a now-faded La Nina climate system, and a stronger-than-normal polar vortex that kept cold from the south largely bottled up.

Elsewhere in the report, Niwa pointed to some striking differences in rainfall around New Zealand: while totals were higher than 149 per cent above normal in eastern parts of Northland and Waikato, they just 50 per cent of normal for parts of inland Whanganui, Gisborne and eastern Bay of Plenty.

June also happened to mark the 53rd month since New Zealand registered a month with a below-average temperature – a trend generally down to a warming climate.

Niwa scientists were now redefining what was “normal” by bringing the 30-year baseline forward to 1991 to 2020.

The switch won’t change any actual weather measurements, but due to our warming planet, it will mean the new temperature normal will be higher than previously.

Looking ahead to the next three months, Niwa has predicted temperatures were “very unlikely” to be colder than average, although occasional cold snaps and frosts could still happen.

Above-average temperatures were most likely for the east and west of the South Island, with about equal chances for above-average or near average temperatures elsewhere.

But more heavy rainfall events were also possible over the coming months – especially in eastern areas – and some of them could be fuelled by “atmospheric rivers” connecting to the subtropics, which powered Canterbury and Gisborne’s recent deluges.

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