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Longest continuous marine heatwave recorded in Bay of Plenty

A school of Jacks in a murky, tropical ocean

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Beachgoers might enjoy warmer waters but they can be harmful for the likes of fish (file image).
Photo: 123RF

Marine heatwave conditions in Bay of Plenty have lasted a year – the longest continuous marine heatwave so far recorded for Aotearoa.

Although coastal water warming may seem great for beachgoers, marine heatwaves can adversely affect ocean life, including kelp forests, fish and marine mammals, making ocean data essential for good management.

The warming was part of a record year of upper North Island marine heatwaves, according to Moana Project oceanographer Dr Robert Smith.

Dr Smith, who is from Otago University, said the Bay of Plenty event started exactly a year ago today and in that time sea surface temperatures have remained on average 1.6 Celsius degrees warmer than the long-term average.

The conditions lasted through winter when temperatures were 2.4deg C above normal.

“This warming is unprecedented over at least the past 40 years and is not only affecting the surface. Moana Project temperature observations collected in collaboration with commercial fishers show that noticeable warming has extended to depths of at least 60 metres,” Dr Smith said.

The warming was part of a wider pattern that has affected the entire upper North Island over the past year, resulting in records being broken.

Waters off Cape Reinga / Te Rerenga Wairua experienced marine heatwave conditions for more than 95 percent of the time in the last year, and off Raglan and Taranaki marine heatwaves lasted more than 80 percent of the time. In the Hauraki Gulf, 274 days had marine heatwave conditions, which was three-quarters of the year.

Cape Reinga

The waters off Cape Reinga in Northland had marine heatwave conditions for more than 95 percent of the time in the last year.
Photo: Phillip Capper CC BY 2.0 via Wikimedia Commons

The long running marine heatwave conditions were likely caused by a combination of factors.

These included recent changes in ocean currents that transported heat around northern New Zealand, persistent La Niña conditions in the Pacific Ocean, and broad-scale warming of the upper-ocean around New Zealand driven by climate change.

Ocean temperatures affect not only marine life, but also the weather generally.

MetService head of weather communications Lisa Murray said marine heatwaves could make rain events worse.

“A warmer sea heats the air travelling over it and because warmer air can mean more moisture, we can get more rain.”

Bay of Plenty Regional Council’s general manager of integrated catchments, Chris Ingle, said having robust scientific information helped to guide good decision making.

With the current La Niña predicted to continue into early 2023, the ongoing marine heatwave conditions were likely to last over summer.

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