This map shows forecast sea temperature anomalies for January 2023 - with orange showing seas 0.5C above average, red 1C above average, and dark red 1.5C above average. Image / Niwa
New Zealand may be in for another major marine heatwave this summer – possibly as severe as two of the biggest we've ever recorded here. Science reporter Jamie Morton explains.
Remind me again, what's a marine heatwave?
More or less what it sounds like.
Just as they measure heatwaves on land, scientists have ways of defining prolonged and unusual warm spells off our coasts – something becoming unfortunately common under climate change.
Niwa considers a marine heatwave as sea surface temperatures (SSTs) that stay in the warmest 10 per cent of historical observations for at least five days.
Once that's happened, they categorise them by intensity: and a major event that powered the hottest summer we've ever sweltered through, in 2017-2018, was deemed "severe".
Soaring sea temperatures contributed to scorching days and nights, packed beaches, wildfires, early grape harvests and widespread glacier melt – but also cascading losses in mussel and kelp beds, and tropical fish drifting into normally colder climes.
More recently, another event developed last spring, when SSTs quickly rose to 1.1C to 1.4C above average – and more than 3C around some parts of the country – helping round out what was New Zealand's warmest year on record.
Remarkably, that marine heatwave was still continuing in some places – and seas in the Bay of Plenty have been persistently warmer for the better part of a year.
Marine heatwave conditions recently recorded in the Fiordland region – where sea surface temperatures have soared to 5C above normal – have meanwhile proven the most impactful seen.
Earlier this year, scientists were alarmed to discover millions of sea sponges there had turned from velvet-brown to bone-white – making for one of the worst bleaching events ever documented among sponge species anywhere.
Even throughout winter, our seas have run abnormally warm - surface temperatures over July ranged from 0.5C to 1.3C above average – and this trend was only expected to pick up as we edged closer to summer.
What's happening right now?
"To be honest, it's easier to count the regions that aren't currently experiencing marine heatwaves than the ones that are," Niwa meteorologist Ben Noll said.
"The only regions where this isn't happening are offshore Hawke's Bay, Wairarapa, South Canterbury and Otago.
🌊 The @niwa_nz Sea Surface Temperature Update for Sep 2022-Feb 2023 suggests that Aotearoa New Zealand's coastal waters have an elevated risk for marine heatwave conditions this summer.
— NIWA Weather (@NiwaWeather) September 15, 2022
This pattern is expected to lead to above normal glacial melt 🏔️https://t.co/HirHdFuYXH pic.twitter.com/CNLNI92Dkv
"This is something that's been ongoing for more than six months now in Auckland, Coromandel, Bay of Plenty, Waikato, Taranaki, Tasman and off the West Coast of the South Island, as well as in parts of the eastern Tasman Sea."
As of September 13, SSTs in New Zealand's six "climate regions" ranged from 0.6C to 1C above average, relative to the 1991-2020 long-term average.
"If we were to compare that with the same day last year, around the North Island, SSTs are two-tenths to six-tenths of a degree warmer – so it's a pretty concerning picture."
What happens next?
According to the latest guidance, the signal for marine heatwave conditions for the months ahead was even stronger than the state of play last September.
"That doesn't guarantee we'll have a marine heatwave as strong as last year – but it certainly gives you an idea of the direction of travel," Noll said.
All things considered, New Zealand was potentially in for another event comparable with 2021-22, and also 2017-18.
Drawing on international models and a wide range of data sources – from satellite monitoring and a fleet of robotic instruments called Argo floats, to observations taken from ships – Niwa issued new forecasts each month.
According to mid-range forecasts, for instance, the agency was picking SST anomalies of nearly 1C near Coromandel town over the first month of 2023 – along with 1.7C in Golden Bay, 0.6C in Opotiki and 1.4C in Pelorus Sound.
"In the near-term, we're expecting to see a substantial uptick in SSTs between October and November, then another pretty big rise between November and December, and basically holding firm as we go through January and February," Noll said.
"While some places can actually see the peak of their marine heatwaves in December, it's usually not until the end of summer that we see these events begin to fade."
Does this mean our summer is going to be extra hot?
It's worth noting that last summer was our fifth warmest ever – and 55 different locations saw record or near-record average temperatures.
And there's one major ingredient that's returning for a rare "three-peat" - La Niña.
While forecasters are ever-careful in picking what this climate system will mean for different regions, unusual widespread warmth is a virtual certainty this summer.
Even over spring, Niwa was picking "very likely" above-average temperatures for most regions.
Marine heatwaves, which often coincide with La Niña here, would only crank the heat higher over the holiday period and beyond.
"In northern regions, particularly, we can all remember that humidity and higher overnight temperatures from last summer, which can really be oppressive for some people," Noll said.
"And if you're swimming at the beach in somewhere like Northland, you might close your eyes and think you're in the subtropics."
Noll expected scientists would be keeping a close eye on how our marine species fared through yet another high-stress event – as well as the health of our fast-melting glaciers.
What's driving these heatwaves?
They're typically the result of a combination of factors.
The 2017-18 event – something scientists said would have been unusual in even 30 years' time - was put down to an oceanic and atmospheric stew of La Nina, blocking anti-cyclones centred over the Tasman Sea, fewer low-pressure systems, and a strongly positive Southern Annular Mode.
Some of those ingredients would contribute again, as would the influence of some dramatic ocean temperature anomalies in the wider region New Zealand lies in.
Hovering above all of this is the giant elephant in the forecasting room: climate change.
Our seas have been warming by an average 0.2C per decade – and that pace of heating is quickening.
On top of what we're already witnessing, Kiwi scientists have warned that average sea temperatures could rise by 1.4C within four decades – and almost 3C by the century's end.
That would mean that, by mid-century, we could be facing 260 days of marine heatwaves per year – and 350 days by 2100 – compared with the 40-odd days we see now.
For some regions such as the southern tip of the South Island, recent Niwa-led research found, there was a high chance that marine heatwaves could start to last more than a year.
Again, those warmer seas meant warmer temperatures on land, and shrinking snowlines – but also more energy for the subtropical-flavoured storm systems we've seen over a record-wet winter.
"All of this is yet another piece of evidence that Mother Nature is really ringing the alarm bells here," Noll said.
"The oceans have kind of been saving our bacon by swallowing up a lot of the excess heat we've been generating.
"Unfortunately, we, and the generations that follow, will pay the price with a feedback loop of warmer seas and warmer land temperatures."
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