Auckland has been drenched with nearly an entire year’s rainfall in fewer than five months, with totals running almost three times above average.
The latest weather station data from Auckland Airport shows a whopping 1018.6mm of rain has fallen there since January 1, just over 90 per cent of the annual average.
Had it been an average year for rain, the station would have recorded only about 381mm so far – which was still more than the 257mm and 250mm it logged at this time in 2022 and 2021 respectively.
Much of the year’s extra rain has come in one-off extreme events, notably the one-in-200-year Auckland Anniversary Weekend deluge that contributed to the city’s wettest month in at least 170 years.
A whopping 280mm fell on Albert Park on January 27 - 211mm of that coming in under six hours.
“Certainly, 2023 is the wettest start to a year [in Auckland] by a considerable margin,” MetService meteorologist Andrew James said.
The only years that happened to come close – but still by a long way - were 2011 (600mm at this time of year) and 2017 (634mm).
The start of the year had been especially gloomy for Aucklanders, who received a paltry five hours of daily sunshine over January, amid what one forecaster called a “bummer summer” of relentless rain and humidity.
To date, however, Niwa’s station at Motat had recorded about 837 sunshine hours, which wasn’t far off the end-of-May cumulative average of 909 hours.
Elsewhere in the north, rainfall figures were similarly staggering.
Whangārei Airport received 1327.8mm this year, roughly three times its mean of 494mm for the year to date.
Tauranga Airport’s station had recorded 1043.8mm, about twice its year-to-date mean of 527.6mm.
Hamilton Airport had logged 665mm, versus a mean of 442mm.
Big totals were also registered at New Plymouth Airport (665mm against a mean of 470mm) and Whanganui (558.2mm compared with a 341mm year-to-date mean).
On Saturday, Whanganui recorded its second wettest May day in more than 40 years of records, with 52.4 mm.
At the very top of the North Island, a Cape Reinga station had so far measured 739.4mm, more than 250 per cent of its 285.5mm mean.
Further south, however, the year had been a hot and dry one.
Invercargill’s rainfall to mid-May only amounted to about 374mm, which was below average for the city.
James singled out one culprit in particular for the north’s sopping-wet start to 2023: La Nina.
The ocean-driven climate pattern – known for bringing muggy, wet conditions to the northeast but a drier flavour to the south – has meddled with our weather since the start of the decade.
“[The rain has] been mostly driven off the back of La Nina, which has made it easier for these systems to come out of the north toward us,” James said.
“We’ve had lots of very warm air coming over New Zealand as a result of La Nina, and warmer air can hold more moisture than cooler air, so we’ve seen significant rainfall right across the north, from Taranaki upward.”
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.
Rain-makers had also been 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.
While the La Nina system itself dissipated earlier in autumn, its effects were still lingering in an ocean-atmosphere lag that meteorologists have partly attributed to this month’s soakings.
Coming in its place was its counterpart 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.
Earlier this month, scientists suggested the sun-blocking, ocean-cooling effects of Australia’s catastrophic bushfires in 2019-2020 might have triggered the three-year La Niña.
Another new study found that strong La Nina and El Nino events have been becoming more frequent under climate change.
While theories about last year’s Hunga-Tonga eruption causing the extra rain have been shared across social media, climate scientists have told the Herald that any direct rainfall effect – namely increased rainfall under its tropospheric water vapour plume – would have played out within weeks, early last year.
Meanwhile, forecasters are picking a sunny reprieve for many regions in the north – but not before another low-pressure system makes its way across tonight and tomorrow.
“Many parts of the North Island can expect rain or showers, very likely with accompanying thunderstorms, between today and tomorrow,” MetService meteorologist Mmathapelo Makgabutlane said.
Severe weather watches for heavy rain are in place across the Coromandel Peninsula, Bay of Plenty and Tairāwhiti-Gisborne and the Wairoa District.
Although rainfall amounts may not reach warning criteria over widespread areas, sodden grounds mean even lower accumulations could bring the risk of impacts such as surface flooding.
While the eastern North Island has some showers into Wednesday, the rest of the island will feel the welcome influence of a high-pressure system.
“This high pressure brings a temporary clearance in weather that people in the North Island have been waiting for over a couple of weeks now,” Makgabutlane said.
“It probably won’t last long enough to fully dry the washing, but it will be a welcome break from what has been a very wet month so far.”
That clearer weather continues for the rest of the working week; however, by Friday night, the upper North Island could have the odd shower again.
Looking ahead, MetService meteorologists are keeping an eye on another low-pressure system over the north Tasman Sea that may drift southwards later this weekend.