New Zealand's summer was shaped by an "extremely active pattern across the country", with lower than normal air pressure and a weak to moderate La Niña helping drive months of volatile weather, according to Earth Sciences New Zealand’s seasonal climate summary.
The report said the country experienced “generally westerly or southwesterly winds across the North Island and southeasterly winds for the South Island" and was dominated by a cyclonic flow.
This setup fuelled severe storms, record-breaking rainfall and destructive winds, affecting communities from Northland to Canterbury, and according to a climate scientist, offers a window into New Zealand's future in a warming climate.
January's tropical low and February's storm delivered some of the most extreme rainfall totals in New Zealand's history.
On January 21, Tauranga Airport recorded 274mm, its highest rainfall on record, while Whitianga, Hicks Bay, Te Puke and several Coromandel locations also received their heaviest or near heaviest daily totals.
The impacts of January's weather were deadly. Nine people were killed, six in a landslide at a Mt Maunganui holiday park, two when a landslide hit a house in Papamoa, and one when his vehicle was swept away in flooding near Warkworth.
A storm in mid February slammed large parts of the central North Island before looping back and impacting the lower North Island, the East Coast, and Canterbury.
State highways were damaged, homes were evacuated due to flooding and one man was found dead in floodwaters between Pirongia and Ōtorohanga.
Akaroa received 563mm which the report described as "333% of its normal summer rainfall" – mostly from the storm that impacted the country in mid-February.
Nationally however, the average temperature was just 0.1C above normal, making it an "average summer". Overall, the extremes were stark.
Parts of Gisborne and Coromandel were recorded as "well above average (more than 1.2C above average)", while the Southern Alps and parts of Otago were "below average (0.51-1.2C below average).
Napier recorded the country's hottest temperature, 36.8C, on January 11 when a warm air mass brought what the report said was "the hottest weather of the summer" to many locations.
However, Winchmore in Canterbury recorded its coolest summer on record, sitting 1.8C below average.
The report found five of the six main centres had above normal rainfall, with Tauranga the wettest, recording "well above normal" at 517 mm. Wellington and Christchurch also recorded "well above normal" rainfall, at 157% and 175% of their typical summer total.
Auckland and Tauranga were the only main centres with above‑average temperatures.
Earth Sciences NZ climate atmosphere and hazards manager Nava Fedaeff told 1News the season's weather mirrored future projections of a warmer climate.
"We live in a warmer world already, so all of our weather is being impacted by climate change. It’s not just the extremes, it’s our everyday weather too," she said.
"The projections for our future climate show that we are expecting more frequent and more intense rainfall events, and we certainly had a lot of those in New Zealand."
It was difficult to say immediately following a weather event how much climate change contributed butattribution studies, which take more time to complete, could provide a clearer picture. After Cyclone Gabrielle, one such study found the event produced about 10% more rainfall overall, with peak rainfall rates 20–30% higher than it would have been in a pre-warming climate.
Fedaeff said this was what New Zealanders should expect – storms that may occur naturally but were now more intense, pushing weather over the threshold from a wet day to a damaging flood.
"Maybe not having that 20% to 30% extra rainfall would have been a wet day, but not necessarily flooded over the bank, so that's when you start to hit those tipping points is when you start to see some of those impacts."
Warm sea surface temperatures also played a role this summer, as subtropical systems drew their energy from latent heat, or evaporation from warm oceans.
"We're surrounded by oceans, so we're surrounded by this fuel essentially," she said.
"They're definitely a piece of the puzzle when we're looking at what contributes to higher rainfall amounts because we're just kind of putting more fuel on the fire."
She said climate change acted as an "exacerbation" of New Zealand's existing weather patterns, with increasing "seesawing" between dry and wet or cold and warm.
"We are seeing a lot of that now, and the projections show that is going to become more in the future."
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