More on the Tasman marine heatwave

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Unprecedented rise in South Island ocean temperatures

Radio NZ,

1 February, 2019

As the country grapples with some of the hottest weather on record, new research has found ocean temperatures around the South Island are also reaching unprecedented levels.

Photo: 123rf.com

New Zealand, Australian and US scientists have just published a study on last summer's heatwave, finding sea water temperatures around the South Island turned almost sub-tropical.

The Tasman Sea warmed on average by 3.7C above normal to reach 20.6C, with fish normally found in the tropics straying further south than they are supposed to.

University of Otago physical oceanographer Dr Robert Smith said while this may be good for recreational fishers, it pointed to the disruption of ecosystems that could be devastating to some species.

"Reports started coming in over summer from local fishermen. On average over the last 10 years they've caught one or two kingfish per summer season.

"But they started catching one or two kingfish per day off the Otago coastline and there are even a couple of days where they've caught 10 to 12 kingfish."

Kelp forests along Otago's coastline, which provide habitat, food and nursery grounds for fish and species dwelling on the sea floor, were also unusually absent last summer.

"If you were a surfer, you could wear board shorts instead of a wetsuit in Dunedin - almost unheard of - with sea temperatures around the 20-21 degree mark at St Clair.

"This multi-disciplinary work has shown that regional heat waves can develop rapidly and have widespread impacts on ecosystems."

The study found that the heat wave was driven by the dual effects of warm atmospheric air and low wind speeds, which would usually have a cooling effect on the sea.

Dr Smith said a parallel study carried out by Australian-based scientists showed last year's marine heat wave would likely not have happened without man-made influence.

Should conditions be at all like they were last summer - as they appear well on track to be - sea temperatures could rise further.

"This is what some children today can expect to see in their lifetimes," Dr Smith said.

The study, just published online in Environmental Research Letters, found the summer of 2017-2018 heat wave was the most intense on record when it came to ocean temperatures.

The New Zealand, Australian and US collaborative study was led by climate scientist Jim Salinger, and involved scientists from Otago's Marine Science, Geography and Surveying Departments, as well as from Victoria University and the National Institute of Weather and Atmospheric Research (NIWA).

It used data sets by scientists from Victoria University of Wellington, NIWA, the National Oceanic and Atmospheric Agency, the University of Otago and the University of New South Wales.