Marine heatwave conditions that began in the Bay of Plenty one year ago today are lingering on – making for a dire new record and a taste of what New Zealand can expect under climate change. Photo / Supplied
Marine heatwave conditions that began in the Bay of Plenty one year ago today are lingering on – making for a dire new record and a taste of what New Zealand can expect under climate change. Photo / Supplied
Marine heatwave conditions that began in the Bay of Plenty one year ago today are lingering – setting a dire record and a taste of what New Zealand can expect under climate change.
In the longest continuous marine heatwave observed here, sea surface temperatures in the region have been running at around 1.6C above the long-term average.
Instead of cooling over winter, water temperatures instead climbed to 2.4C above normal.
“This warming is unprecedented over at least the past 40 years and is not only affecting the surface,” University of Otago oceanographer Dr Robert Smith said.
Data collected with help from commercial fishers as part of the collaborative Moana Project showed that noticeable warming had reached depths of at least 60 metres.
And this long run of unusually balmy seas is only expected to continue through summer, when forecasters predict widespread heatwave conditions for our coastal waters.
Niwa’s latest seasonal forecasts suggest sea surface temperatures stretching between Tasmania and the Chatham Islands could be averaging 1.1C above normal in January – with many coastal spots likely to see particularly high values.
The warming was part of a wider pattern that has affected the entire upper North Island over the past year.
“While the Bay of Plenty is experiencing the longest marine heatwave that we now have on record, many areas in the North Island have broken local records too.”
Waters around Northland recorded these conditions for about 95 per cent of the past year, while heatwaves had persisted off Raglan and Taranaki for around 80 per cent of the time.
“In the Hauraki Gulf, 274 days had marine heatwave conditions, which is 75 per cent of the year.”
The scientific community 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.
As with other events over recent years – notably one that fuelled our record-hot summer of 2017-18 – there was a combination of drivers at play.
These include recent changes in ocean currents that transported heat around northern New Zealand, weather and climate patterns in the Pacific Ocean, and broad-scale warming of the upper ocean around New Zealand driven by climate change.
“In the last year, we’ve seen consistent patterns of blocking high-pressure systems east of New Zealand, which is associated with the ongoing La Nina, but also another climate phenomena called the Southern Annular Mode.”
This indicator - a ring of climate variability that encircles the South Pole, but stretches far out to the latitudes of New Zealand – had trended towards a positive phase, favouring relatively light winds and more settled weather in our neighbourhood.
“These two things together have likely disrupted the normal westerly airflow over New Zealand, meaning relatively weak winds, less vertical mixing in the ocean and reduced heat loss – all which have helped set up extreme ocean temperatures.”
The contribution of climate change – which has been pushing up sea surface temperatures at a rate of about 0.1C to 0.2C per decade, and quickening – was only worsening the picture.
Niwa scientists have also warned that average sea temperatures around New Zealand 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.
The Bay of Plenty has been under marine heatwave conditions for a year today. Source / MetOcean Solutions
“What we’ve seen around New Zealand this last year is a really clear window into what the average year is meant to look like at the end of the century; to some extent, we’ve got that now.”
While warmer seas might seem great for beachgoers, marine heatwaves could wreak havoc on ocean life.
Over recent summers, fishers in Otago and Southland have been increasingly reporting species like kingfish and snapper – traditionally considered North Island fish – but the greatest risk was to marine life that couldn’t shift.
Scientists were alarmed this year to discover millions of sea sponges in Fiordland had turned from velvet-brown to bone-white – one of the worst bleaching events documented among sponge species anywhere.
Our largest salmon exporter, New Zealand King Salmon, was also recently forced to close farms in Pelorus Sound after a heatwave led to 1300 tonnes of dead salmon being sent to landfill and resulting in local job losses.
Sea surface temperatures around New Zealand are consistently high, with several areas under prolonged marine heatwave conditions. Image / MetOcean Solutions
Bay of Plenty Regional Council’s general manager of integrated catchments, Chris Ingle, said good scientific information helped to guide good decision-making.
“To build climate resilience, we value sound data from the Moana Project,” he said.
“This information helps us, as regulators, manage the pressures on the marine environment.”
Ocean temperatures affected not only marine life, but also the general weather on land.
Soaring sea temperatures during 2017-18 contributed to scorching days and nights, packed beaches, wildfires, early grape harvests and widespread glacier melt.
