It also showed how the dramatic climate trends of 2023′s opening months – a north-and-south, wet-and-dry contrast dubbed a “tale of two summers” by one Niwa meteorologist – had only persisted into autumn.
Totals for Auckland, which received 90 per cent of its average annual rainfall in a few months, were running at about three times the 30-year baseline.
Much of that had come in one-off extreme events - notably the one-in-200-year Anniversary Weekend deluge that dropped several hundred millimetres of rain on suburbs over 24 hours and contributed to the city’s wettest month in at least 170 years.
A whopping 280mm fell on Albert Park on January 27 – with 211mm of that coming in fewer than six hours.
“Statistically, the Auckland event is a paradigm shift in the way think about extremes.”
As he expected, the data also confirmed an extraordinarily wet 2023 for Rotorua, where, over five months, rainfall had been sitting at record levels for about half the period.
By mid-May, local cumulative totals had already climbed past 1000mm – or roughly three quarters of its annual average.
Further east, around half of Gisborne’s annual average rainfall had dropped by summer’s end, on the back of a markedly wet January – and the more than 185mm delivered by Cyclone Gabrielle alone.
It’d also been a “miserable” start to the year in New Plymouth and Wellington, Melia said, with cumulative totals for both centres hovering around the 500mm mark.
While Nelson’s total was also running above the annual average after big downpours over recent weeks, further south, the picture was markedly different.
“Dunedin and Invercargill seem to have had a great summer – and it’s hard to imagine a more perfect 2023 for Queenstown, weather-wise.”
Although Christchurch’s January was also on the drier side, the last few months had been much wetter.
Melia said 2023′s rainfall trends could be broken down to a “pyramid of factors”.
At the bottom lay several background climate influencers: notably the ocean-driven La Nina, which has been meddling with our weather since the start of the decade, and notorious for bringing muggy, wet conditions to the northeast but a drier flavour to the south.
The stage had already been set by the negative phase of another climate phenomenon called the Indian Ocean Dipole – a big cause of 2022′s record-wet and warm winter – and local pressure systems unfortunately positioned by a positive Southern Annular Mode.
Then, in January, New Zealand saw rain-makers intensified by regional marine heatwaves; the damp influence of the equator-circling Madden Julian Oscillation; and ongoing climate heating that’s loading more moisture into the atmosphere.
“In summer, moisture potential can be the greatest if the other ingredients are present,” he said.
“These factors in turn fed into the Auckland event and Cyclone Gabrielle.”
While the La Nina system itself dissipated earlier in autumn, its influence was still lingering in an ocean-atmosphere lag that meteorologists have partly attributed to recent soakings.
Coming in its place was its counter-part climate pattern El Nino – which spelt a potentially record-hot year for the planet, but a shift to cooler, drier conditions across much of New Zealand.
“The effects of El Nino have already arrived in the coastal eastern pacific waters,” Melia said.
“However, in our side of the Pacific, the departure of La Nina warmth is slow, and indeed, it will take months to a year for El Nino to make its way westward across the Pacific.”
Niwa meteorologist Chris Brandolino said it remained to be seen just how much El Nino might flip the climate patterns of our last three years under La Nina.
“But I don’t think it’d be controversial to say that those areas that have seen an abundance of rain over the past six months are probably going to be on the drier side for the next six.”
