Weather: Climate change 'not the only reason' why New Zealand recorded warmest June on record - NIWA forecaster

Last month was officially New Zealand's warmest June on record - news that might set alarm bells ringing for the environmentally conscious. But an expert says climate change is not the only reason behind the milder temperatures this winter, with several other factors at play.

Last week, a polar blast from the Antarctic sent temperatures plummeting as bone-chilling winds swept the nation, causing heavy snowfall in several areas. But the sudden cold snap had little impact on the higher-than-average temperatures that month, says Nava Fedaeff, a forecaster at the National Institute of Water and Atmospheric Research (NIWA).

"It was one of the warmest months of all months on record, just looking at the anomaly values," Fedaeff told The AM Show on Tuesday morning. 

When analysing anomalies - months that saw a deviation from the average temperatures expected at that time of year - experts determined that June was the 13th warmest month on record since 1909, Fedaeff said. It is only the 13th time since 1909 that a month recorded a temperature anomaly of more than 1.9C above average.  

But this week will see a return to wintery weather, she added, with cool air set to swathe the west of the South Island on Tuesday before shifting to the north on Wednesday. The cold front will spell snow for the lower South Island, with rainfall also on the cards for a number of regions, particularly along the west coast.

It's a dramatic departure from the abundance of northeasterlies last month - winds that played a key factor in the balmy beginning to the winter season. 

"That was one of the key reasons why June was warmer - more northeasterly winds. We had high pressure to the east of us, so that drives that northeasterly flow. One of the drivers is we've had a pretty strong polar vortex - that sounds cold, but the polar vortex is winds that circumnavigate Antarctica, high up in the atmosphere," Fedaeff explained.

"They were locking a lot of that cold air in place, so there weren't a lot of cold southerly outbreaks we would typically associate with winter. 

"That change at the end of June, when Mother Nature opened the fridge door - that's when we got that southerly blast."

But Fedaeff says a number of factors contributed to last month becoming the warmest June on record, including the plethora of northeasterly winds and the polar vortex - meaning climate change isn't solely to blame.

"We already live in a warmer climate… We live in a climate change mold, you would say. It affects every single weather pattern. So yes, that is something that bumps us up, gives us the warmth, but that's not the only reason [sic]," she told The AM Show.

"We had the northeasterlies, we had the strong polar vortex, we had some subtropical storms coming down that brought a lot of humid air, so [there was] a lot of warmth from a lot of different areas - but [climate change] is part of the puzzle, absolutely." 

According to NIWA, the nationwide average temperature last month was 2.0C above average. This is just the 13th occasion since 1909 that a month achieved an anomaly of more than 1.9C relative to the 1981-2010 average. Temperatures were above average (0.51-1.20C above average) or well above average (more than 1.20C) throughout the country, with 24 locations observing their warmest June on record. 

Although rainfall was above normal or well above normal for eastern parts of Northland, inland Bay of Plenty, eastern Waikato, Wairarapa, northern Canterbury, southeastern Otago and western Southland, rainfall was below average or well below average for parts of Central Otago, South and Mid Canterbury, Nelson, Tasman, inland Whanganui, Gisborne and eastern Bay of Plenty. 

"June 2021 mean sea level air pressure was above normal to the east of Aotearoa New Zealand. This was associated with more northeasterly airflows than usual over the country," NIWA said in its climate report for June.

"The prevalence of these airflows, 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."

Further facts:

  • the highest temperature was 22.0C, observed at Hastings on June 26, and Leigh on June 19
  • the lowest temperature was -7.4C, observed at Middlemarch on June 19
  • the highest one-day rainfall was 162mm, recorded at Arthurs Pass on June 25
  • the highest wind gust was 191km/h, observed at Cape Turnagain on June 28
  • of the six main centres in June 2021, Auckland was the warmest and sunniest, Tauranga was the wettest, Christchurch was the coldest and driest, and Dunedin was the least sunny
  • the sunniest four locations in 2021 so far are Taranaki (1331 hours), Bay of Plenty (1299 hours), Marlborough (1291 hours) and Hawke's Bay (1277 hours).