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Strong “marine heatwave” conditions are being observed around the North Island’s northern and eastern coasts.
The abnormal warmth is expected to add to air temperatures and humidity – especially after a shift to “La Nina-like” climate patterns from the New Year.
While warmer water might be welcome for beachgoers, marine heatwaves have been hitting our vulnerable ocean ecosystems hard.
Strong marine heatwave conditions which have formed around northern coasts are set to crank up air temperatures this summer, while making for muggy days and nights.
National Institute of Water and Atmospheric Research (Niwa) meteorologist Chris Brandolino said marine heatwave conditions – prolonged periods of abnormally high sea temperatures – were being observed off the north and east of the North Island.
That unusual ocean warmth, ranging from 1-3C above normal, was expected to intensify over the next few days as a period of settled weather and strong sunshine heated the surface waters and further hiked up temperatures on land.
This feedback loop, a pattern that could persist for as long as until May and delay autumn coolness, meant people in coastal areas in northern New Zealand were more likely to experience extra-hot days and increased humidity over summer, Brandolino said.
Brandolino said marine heatwave conditions would add to the uncomfortable humidity these arriving systems delivered to northern areas – and potentially help rainmakers maintain their intensity for longer.
Strong marine heatwave conditions are being observed around the North Island's northern and eastern coasts. / The Moana Project.
“If you have higher sea surface temperatures, you’re chucking more water vapour into atmosphere, which is fuel for rain,” he said.
“So, this may well contribute to what moisture there is in rain-bearing systems coming from the north.”
Right now, he expected the heatwave conditions would be noticeable to beachgoers in northern spots.
“If you’re in Wellington, temperatures along the south coast are closer to 16C, which is a degree above average, but still chilly,” he said.
“But for people in Northland, Auckland or Hawke’s Bay, where sea temperatures are running at 19C, those anomalies will be noticeable.”
While warmer water might sound ideal for an evening dip, marine heatwaves have been anything but welcome for vulnerable ocean species.
Over recent years, scientists have observed mass die-offs among sea sponges and life-supporting kelp forests, along with feeding and breeding disruptions to animals like little blue penguins and our resident giant blue whales.
Otago University oceanographer Dr Robert Smith expected the predicted return of La Nina-like conditions could pose more such implications.
“Just as we’ve seen in other summers, we could well have weather systems that push the coastal ocean into a more extreme state – or higher than the temperature anomalies we’re observing.”
These maps show sea surface temperatures and anomalies as at mid-December. Image / Niwa
Scientists would be closely monitoring abnormal temperatures deeper in the ocean, where some of the most dramatic impacts have been observed.
“We’ve certainly seen that happen in 2021-22 and again in 2022-23 – and those were both La Nina summers.”
Studies suggest marine heatwaves will grow longer, stronger and more frequent around New Zealand under climate change, which has seen our seas warm by an average of 0.2C per decade.
Scientists warn this trend is worsening fast: average sea temperatures could rise by 1.4C within four decades and almost 3C by the century’s end.
That would mean by the middle of the 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.
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
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