A fisherman tries his luck at Stokes Point Reserve in the gloomy Auckland weather on January 3. Photo / Jason Oxenham
A fisherman tries his luck at Stokes Point Reserve in the gloomy Auckland weather on January 3. Photo / Jason Oxenham
It's a tale of two summers: one, New Zealand's third-warmest on record; another, a mess of cool days in the south, heat in the north, and widespread northwesterly winds.
In 2018-19: a marine heatwave, a 40-day dry spell in Nelson and another heat-streak that enveloped the country halfway through theseason.
In 2019-20: a climatic jumble and a colder, stormier ride for many regions – with the obvious exception of a parched north, driven to the verge of drought by a persistent ridge of high pressure parked to the northwest in the tropics.
Even Wellington, notorious for its often dismal summers, saw 10 days of temperatures over 25C last season; this time round, there hasn't been a single day since December 1 that's cracked that number.
The SAM spent much of the last summer in a positive phase, bringing relatively light winds and more settled weather over the country, together with enhanced westerly winds over the southern oceans.
In Wellington - where fog cancelled flights today - temperatures haven't crossed 25C all summer. Photo / NZME
This positive SAM had also played a part in creating the second marine heatwave in as many years – and those warmer waters helped to keep temperatures on land running hot.
This season, however, meteorologists have been watching in fascination at a SAM that's kept largely negative since spring.
What does negative SAM mean?
Think increased westerlies and more unsettled weather, with eased windiness and storm activity over the southern oceans.
The curtain-raiser for this enduring negative SAM was what Niwa forecaster Ben Noll called an "oddball driver" – a rare stratospheric warming event high over Antarctica in spring, which exerted a chilly influence over much of the hemisphere.
"That tipped us off that there was going to be some wacky weather coming over the next few months."
What had been a generally clean pattern of drier, warmer conditions took a cooler, wilder turn in September and October, but a large bout of heat the next month resulted in the hottest November on record.
"It's been kind of a roller coaster. We had that warm November. Then in the first half of summer, we've had winds blowing north of west sometimes, and south of west other times."
Another influencer – a climate driver called the Indian Ocean Dipole – had long been stuck in a positive phase, and this had helped brown Eastern Australia to the point of crisis.
"But for us in New Zealand, this positive IOD helped allow many systems to move up the country over the last few months – you could almost say that it kept the door to the Southern Ocean ajar."
Noll pointed out a third factor – the absence of either a strong El Nino or La Nina climate system to give summer a simple, predictable flavour.
Instead, the tropical eastern Pacific remained in a El Niño–Southern Oscillation (ENSO) neutral state – and was expected to right into autumn.
"Last year we had a weak El Nino, which brought warm westerly winds. This time around, it's as if we're here waiting for a conductor to step in and lead the band," he said.
"It's interesting, as, because both that negative SAM and positive IOD are fading, for the first time in several months, we won't have a handful of strong drivers."
What happened next?
Noll expected the cooler temperatures that had coloured January to shift to the warmer side next month.
A persistent ridge of high pressure north of New Zealand has pushed Northland to the verge of drought. Photo / NZME
MetService meteorologist Jake Cope expected to see changeable, "quite messy" weather through to the end of summer – or what he summed up as a "negative SAM kind of vibe".
"It may be a little bit cooler in some places of the south, but not the upper north, because we'll always have that ridge hanging in there, which makes for more settled conditions."
For those wondering about any hidden hand of climate change, scientists say there's nothing yet to suggest that global warming favoured any particular driver.
"These normal variables aren't going away – and as far as we know they are not really changing their characteristics," Victoria University climate scientist Professor James Renwick said.
"It's still a big research question, because if the intensity or the mix of El Nino and La Nina events changed significantly, that would be a big deal for the global climate."
Not so fun fact: Wellington has yet to reach 25 degrees so far this summer. The average is 5 days per summer.
Only two summers have come and gone without a temp at or above 25 since 2000! 😨
One effect that was increasingly expected to play out was rainfall that came with any climate system to become more intense – something that generally owed to higher temperatures driving more moisture in the atmosphere.
And, despite the hodge-podge of summer weather, Noll said there was enough evidence already of a long-term warming trend.
If January happened to finish on a warmer note – as it's expected to in many parts of the country next week – it would be 36 months since New Zealand had seen a month of below average temperatures.
"I think that is the most important stat," he said.
"Maybe we've just got used to this warmer baseline of what we expect during summer, and when we have a step down, it's a bigger shock than we'd have otherwise."
