This colour-coded map shows sea surface temperature anomalies around New Zealand on June 14, with orange representing 1C above average, and dark orange 2C or more above average. Image / Niwa Weather
New Zealand’s waters are still running abnormally warm after years of relentless marine heatwave conditions – and could be taking some of the bite out of the winter chill we’ve been feeling this month, a meteorologist says.
While the arrival of an El Niño climate pattern is expected to help finally cool down our coastal seas, experts say it’s likely to take months longer for temperatures to drop deeper in the ocean, where some of the most dramatic impacts have been observed.
A stew of factors – notably three years of a now-faded La Niña and the rising influence of global climate change – have been pushing our local sea temperatures to extraordinary new records, with wide-ranging implications.
They’ve been linked to heatwave days, glacier melt, earlier grape harvests, mass sea sponge and kelp die-offs, and fish species like kingfish and snapper drifting further south, and disruptions to breeding and feeding among our resident blue whales.
Even as forecasters have been recording bracing June temperatures, the sea surface around most of New Zealand has been running 1C to 2C warmer than what would be normal for this time of year.
MetService oceanographer Dr Joao de Souza said the biggest anomalies were being observed off the southeast of the South Island, where surface temperatures were as high as 3C above average.
“However, there are smaller patches of similarly warm waters in the Wairarapa coastline and the east coast of Northland, as well as the Chatham Islands.”
Niwa meteorologist Ben Noll said temperature data over the month to date suggested this had been partly off-setting cold conditions in coastal areas of the South Island.
🌊 Believe it or not, marine heatwave conditions (🟡) are still occurring in New Zealand's coastal waters.
— NIWA Weather (@NiwaWeather) June 16, 2023
This has likely had a moderating effect on coastal air temperatures so far this winter.
Although it's been chilly lately, no minimum temperature records have been broken! pic.twitter.com/SAmFciSdtp
“That type of spatial pattern showing warmer conditions hugging the coast is linked right up with the sea surface temperature pattern.”
Asked where the warmer sea trends had been the most persistent in recent times, de Souza said the simpler question was where they hadn’t.
“Marine heatwave conditions have been so prevalent over the last couple of years that in most areas there [have been] more days in marine heatwave conditions than days where temperatures were ‘normal’,” he said.
“For example, the area around Cape Reinga had a single marine heatwave that lasted 175 days and the Bay of Plenty had marine heatwave conditions for longer than a year.”
The year had begun with record-breaking anomalies at Stewart Island, the Otago Peninsula and Fiordland, which, over the past year, had experienced only a few weeks of average temperatures.
“[That] means that the average sea surface temperature was a whopping 2.4C above normal,” he said.
“Even the Cook Strait was on average 2C warmer than normal.”
Data collected by local fishing vessels, equipped with sensors as part of the collaborative Moana Project, had also shown some major temperature anomalies had been intensifying at depth.
Our research vessel Polaris II @otago was recently in #Fiordland and equipped with a #MoanaProject Mangōpare sensor from @JulieJakoboski it captured 41 ocean temperature profiles, in near real-time! Here's a 3D data viz using @MATLAB @MetOceanNZ @MBIEsci https://t.co/LTU5nx9iwv pic.twitter.com/kY1oBG8IMD
— Robert Smith (@robowainsmith) May 28, 2023
In the Bay of Plenty, this had been occurring 80m beneath the surface – and as far down as 200m in Fiordland and to 400m around the subantarctic Auckland Island.
Right now, what was being recorded wasn’t “drastically different” to the unusually warm winter ocean temperatures of the last few years.
“The ocean has a larger capacity to store heat than the atmosphere, especially below the surface layers,” he said.
“Therefore, the right weather conditions can trigger a marine heatwave.”
He added that not all the events were the same: some happened to be shallow, while others were deep or completely disconnected from the surface.
“Some are caused by local atmospheric conditions while others are a consequence of ocean currents bringing anomalous warm waters,” he said.
“All are intensified by global warming, and more often than not a combination of such factors is necessary for the strong events we lived in the last few years.”
Last week, a University of Canterbury-led study provided more evidence to show that marine heatwaves had been becoming longer, stronger and more frequent across coastal New Zealand over recent decades – but especially in the latest one.
It found how different regions experienced more marine heatwaves in different seasons - with the greatest increases observed in the Chatham Islands in winter, central New Zealand in autumn, northeastern New Zealand in summer and autumn, and the Bounty and Antipodes Islands in spring.
The study team also pointed out how these events could have severe impacts on near-shore ecosystems and called for better monitoring of these habitats.
In another just-published, lab-based study, Victoria University scientists reported how marine heatwaves could drive “evolution at super speed” among a common sea sponge species forced to adapt to the rapid change.
Many simply couldn’t.
“Our experiments found just over a third of adult sponges died after only 10 days spent at 21C—conditions similar to those in Wellington harbour during the marine heatwave in January 2018,” study co-author Professor James Bell said.
Looking ahead, the shift to El Niño was expected to come with a gradual fall in sea temperature anomalies as the built-up extra heat was redistributed by ocean currents and transferred to the atmosphere through evaporation.
Noll said the El Niño was projected to bring more westerly winds – including a potentially “beefy” system later this month – that would help churn up seas and bring up cooler water from below.
This trend could push down sea surface temperatures by as much as 0.5C over July and August respectively, he said, with the pattern continuing into spring.
De Souza nonetheless expected that the heat that’s been taken into deeper layers in the ocean would still impact New Zealand for months to come.
“And the anomalous warm deep waters in the Tasman Sea will be carried by ocean currents towards our shores,” he said.
“So, although things may seem cooler at the surface, we should still experience warmer-than-normal temperatures at depth.”
“This only highlights the urgency of monitoring the ocean below the surface.”