Marine heatwave: Models point to major event this summer

This map shows forecast sea temperature anomalies for February 2023 - with orange showing seas 0.5C above average, red 1C above average, and dark red 1.5C above average. Image / Niwa

The signal for a historically strong marine heatwave enveloping New Zealand this summer has only increased, with most models now pointing to abnormally warm sea temperatures.

New mid-range forecasts show that, by late summer, sea surface temperature (SSTs) could reach 1.6C above average in Golden Bay, with other anomalies of 1.3C in Marlborough's Pelorus Sound, close to 0.9C at Ninety Mile Beach and off Coromandel town, and 0.6C in Opotiki.

"We're looking at modelling from eight different global forecasting centres to better understand what the risks New Zealand might have with sea temperatures over the next half-year," Niwa meteorologist Ben Noll said.

"Seven of these eight are now signalling a marine heatwave event for our region."

Across that region – stretching from Tasmania to the Chatham Islands - Noll said the modelling suggested an average anomaly of about 1.1C in January.

Compared with what the models had forecast for the region in the past, those values were at the upper-end of projected anomalies, and enough to have widespread implications for land and sea environments.

🌡️🌊 The October 2022 - March 2023 @niwa_nz sea surface temperature update continues to show the potential for a marine heatwave event (🔴) around Aotearoa New Zealand this summer & beyond...https://t.co/0VZK3g8nuk pic.twitter.com/TQga3faEWB

— NIWA Weather (@NiwaWeather) October 13, 2022

"The modelling is basically telling us, this is unusual, and we should be paying attention," Noll said.

"The big picture is that we appear to be on track – but where exactly this marine heatwave is most significant, we'll have to wait and see."

Niwa considers a marine heatwave as SSTs that stay in the warmest 10 per cent of historical observations for at least five days.

While swathes of New Zealand's coast have been experiencing local marine heatwave conditions throughout the year, the event was likely to become a regional-scale event over the warmer months – with an escalation in sea temperatures expected over coming weeks.

"These do take a while to really get established and cause a positive feedback loop with our air temperatures – and right now, we're still only looking at the formative stages."

Noll has already warned this event could prove comparable with some of the worst New Zealand has experienced: including a "severe" one in 2017 and 2018 that helped fuel our hottest-ever summer, and warmest-ever month.

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.

The latest projections were also informed by a wide range of data sources, including satellite monitoring, observations taken from fishing boats and a fleet of robotic instruments called Argo floats.

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.