New Zealand's two-year marine heatwave eases, but likely not for long

For the first time in two years, all of New Zealand coastal water temperatures have dropped below heatwave levels, but it might not be for long.

Scientists from the research collaboration group Moana Project said at least one part of the country's coastal waters had been in a marine heatwave since 2020.

But just before summer, Moana Project lead scientist Joao de Souza said a sudden switch in weather caused a drop in sea surface temperatures, relieving the prolonged marine heatwaves.

"The sharp drop in sea surface temperatures was likely caused by the cold, windy weather we've seen in many parts of the country over the last couple of weeks. This cooled down the surface water," de Souza said,

However, the researchers warned coastal waters were still hotter than normal, with the heat pervading to deeper levels than normal, and it was expected the temperatures would probably rise back into heatwave ranges soon.

De Souza said modelling showed that once there was settled weather with some sunshine the marine heatwaves were expected to reappear.

And even at below heatwave levels the warm temperatures deeper in coastal waters were likely still affecting marine life.

"For marine life, the temperature at depth, throughout the water column, is what is important," he said.

Oceanographer Dr Rob Smith from Otago University said that warm water at depth was being seen in many areas.

"For example, offshore from the Bay of Plenty sea surface temperatures are close to normal but at 200 metres depth water temperatures are more than 2 degrees warmer than what they are supposed to be for this time of year,"

The Moana Project is a collaborative science project started in 2020 and funded by the Ministry of Business, Innovation and Employment to model when and where marine heatwaves will affect coastal and oceanic waters.