Low cloud hangs over Auckland on the first meteorological day of spring. Photo / Dean Purcell
Low cloud hangs over Auckland on the first meteorological day of spring. Photo / Dean Purcell
A freshly-sprung spring is predicted to bring warmer weather everywhere – but not before another forecast drenching for parts of the country.
Temperatures over spring were "very likely" to be warmer than average in the North Island, along with the north and west of the South Island – and "most likely" to be warmer in the southeast, according to Niwa's just-issued seasonal outlook.
But it was tougher to predict how rainy spring would prove, thanks to a "battleground" of pressure features hovering around New Zealand – ranging from the odd sub-tropical low to strong belts of high pressure.
With fewer spring westerlies about, Niwa picked rainfall to be either near or below normal in the west of both islands, near or above normal in the upper north, and most likely normal elsewhere.
In the immediate term, another big rainmaker was forecast to sweep into the South Island tomorrow, with heavy amounts of rainfall likely for southern and central Westland tomorrow – and a further event was possible later in September.
There was also the possibility of a low bringing heavy rain to Northland on Monday.
Conversely, the calm, sunny weather Kiwis have been enjoying for the past few days could become increasingly more common, with periods of high pressure and easterly flows bringing long dry spells, especially in the west of both islands.
"One key distinction between what we've experienced in recent months and the upcoming few months is the likelihood these high-pressure systems will become more frequent over our region," Niwa meteorologist Ben Noll said.
Our weather wouldn't magically snap into these conditions, but rather move toward them amid what he described as a "battleground between drier highs and very wet lows".
"But we can expect those lows to be gradually deflected a little further north than they were during winter – and thus fewer regions may become exposed to those sub-tropical rain-makers."
That could ultimately mean longer dry spells in the South Island – but also occasional bouts of heavy rain in the upper north.
Perched behind the driver's seat was New Zealand's third consecutive La Niña – and Noll said its effects could be yet more pronounced than the past two.
Source / Niwa
Traditionally, La Niña has delivered more north-easterly winds that bring rainy conditions to North Island's northeast, and drier conditions to the south and southwest of the South Island.
Thanks to the north-easterly winds, warmer temperatures also tended to play out over much of the country during La Niña, although there are always regional and seasonal exceptions.
One was 2020, when an odd-ball La Niña event delivered a somewhat unexpected flavour – and became among four of 17 La Niña events measured since 1972 that failed to bring near or above normal rainfall for Auckland.
The similarly "moderate" 2021-22 event behaved closer to script, contributing to a record marine heatwave near the North Island, several close calls with ex-tropical cyclones, and flooding in the east and droughts in the far south.
Over July, a surge in trade winds over the equatorial Pacific, where currently cooler waters were now expected to spread eastward over the next two months, further strengthened the oceanic La Niña signal.
"In the equatorial Pacific, we're now talking [sea temperature] anomalies of 3C to 5C, about 500 metres beneath the surface," Noll said.
"What does that mean? Well, that water will be headed toward the surface over the spring season, and when it does, the oceanic component of La Niña could end up being stronger than in the last two years.
"So, this could prove to be, at the minimum a moderate event – but at the high end, perhaps one of the stronger events that we've seen in the last couple of decades."
Noll said the warmer waters currently surrounding New Zealand – coastal sea temperatures stubbornly remained between 0.5C to 1C above average during August – gave this La Niña a balmier kick-start.
"Swathes of our offshore waters have been in marine heatwave conditions for the last six to nine months," he said.
"If we compare sea surface temperatures between August 2021 and 2022, the entirety of the North Island's offshore waters, and the north of the South Island's, were warmer than this time last year.
Coastal sea temperatures stubbornly remained between 0.5C to 1C above average during August. Image / Niwa
"So, the potential jump-off point as we look toward summer is higher than last year - and we know how last year went."
Last summer proved our fifth-hottest in the books, with temperatures "well above" average – and also rounded off the warmest year ever recorded here.
"While drought is something that isn't likely to be mentioned anytime soon, with incoming high-pressure belts, things should gradually dry out as we go through the season," he said.
"Those key areas we typically watch during La Niña, as we did last year, include Southland and Otago – and it's certainly been warmer, and a bit drier, in Southland relative to other parts of the country."
Although it wasn't clear if global climate change made La Niña-like conditions more likely – IPCC models instead point the opposite direction, toward El Niño – background heating also continued to contribute to warmer patterns.
The winters of 2020 and 2021 each brought record-breaking warmth – and 2022's was again likely to sit among the hottest when Niwa issues its summary this week.
• A live-stream of Niwa's seasonal climate outlook will be posted here from 1.30pm.
