As New Zealand heads toward another marine heatwave, scientists are reporting the water around the country is significantly warmer than it was three decades ago.

Niwa scientists behind a just-published study, which analysed ocean temperature changes over the past 36 years, expect this trend only to continue and have pointed to climate change as an obvious driver.

Oceanographer and study lead author Dr Phil Sutton said the strongest warming near New Zealand had occurred off the Wairarapa Coast, and the weakest along the northeast coast between North and East Capes.

The increased warmth wasn't only at the surface – around New Zealand, the increased warmth penetrated in places to 200m deep, while in the Tasman Sea it reached down to at least 850m.

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"Since 1981 we are talking of warming of about 0.1C to 0.3C per decade," Sutton said.

"That may not sound like a huge amount but slightly stronger warming of about 0.4C per decade off the east coast of Tasmania has resulted in significant changes to ecosystems which has led to concerns of similar impacts in New Zealand."

Sutton and co-author Dr Melissa Bowen, of the University of Auckland, analysed three data sets and applied mathematical equations to determine if the changes were statistically significant.

The longest data set was daily satellite measurements of sea surface temperatures which began in 1981.

The second set was derived from ship-based measurements of the ocean down to 850m depth along two transects, or lines, from Sydney to Wellington which began in 1991, and Auckland to Suva which started in 1987.

The final data set was information from floats used in the Argo programme, an international network of floats in the world's oceans that measure temperature, salinity and currents in the top 2000m of the oceans.

Sutton said there were two areas in the South Pacific region where it was well known that strong warming had occurred: east of Tasmania and in the central Pacific, east of North Cape.

The Australian warming was attributed to current flows pushing warmer water further south, while the central Pacific was affected by the South Pacific gyre – a system of circulating ocean currents - speeding up.

However, Sutton said there is variability from decade to decade that needed to be taken into account.

"The 1990s was a cooler period around New Zealand but it warmed up later in the decade with 1998 the second equal hottest year on record," he said.

Image / NIWA
Image / NIWA

"But since 2000 there hasn't really been any warming off the Wairarapa. There is definitely a warming trend but there is a lot of decadal variability behind it."

Cooler years could largely be attributed to El Niño conditions but the warming ocean trend was affecting the water around New Zealand's entire coast to at least 100m deep.

The most obvious cause was climate change.

"There is nothing here that is out of step with what has been projected to happen," Sutton said.

"We would expect to see this continue – perhaps the warming in the northeast will continue to be weak but you would suspect the Wairarapa Coast to be vulnerable to further change.

"The 30-year time series is long enough to see long-term changes and there is no reason to think this will turn around."

Sutton was hopeful the information will be useful to the other marine scientists, commercial fishing industry and aquaculture.

The study has been published in the New Zealand Journal of Marine and Freshwater Research.

It comes after a record marine heatwave last summer, which pushed sea surface temperatures around the country to between 2.5C and 4C above average through much of December 2017, and helped drive the biggest ever ice loss observed on the Southern Alps.

Incredibly, some localised spots off the West Coast even reached between 4C and 6C above normal.

Niwa scientists are now closely watching another event unfolding.

Over past weeks, average sea surface temperatures had climbed more than 0.7C above the long-term average, with waters being observed around the South Island and to the west, south, and east of the country.

Sea surface temperatures were closer to average near Northland and more broadly to the north of the country.

While they were generally warmer than average, they were still markedly cooler than this time last year, when the record marine heatwave event was underway.