Ocean near New Zealand warming faster than anywhere else, study finds

A study led by top climate scientist Dr Kevin Trenberth and others shows New Zealand's slice of the ocean is absorbing more heat than any other. Photo: Chris Loufte

A study has found New Zealand's slice of the ocean is absorbing more heat than any other.

One of the scientists behind the work says the effect is not just making the sea feel a bit warmer for a dip, but worsening storms like ex-tropical Cyclone Tam, which wreaked havoc with heavy rain over Easter.

Hotter oceans do not just affect marine creatures - they are linked to shrinking glaciers and wilder, wetter storms.

Oceans absorb most of the excess heat that people are pumping into the climate system and without them, air temperatures over land would be hotter.

But the seas are not heating evenly.

A study led by top New Zealand climate scientist Dr Kevin Trenberth and others shows two circular bands of the ocean have been taking up more heat than the rest.

One band is located about 40 degrees latitude south and the other is around 40 degrees latitude north.

"The pattern comes out to be quite striking and what we've found is that the biggest warming [in] the ocean is occurring in the middle latitudes near New Zealand's latitudes and also at comparable latitudes in the Northern hemisphere."

The band with the strongest heating is the southern one where New Zealand sits, because southern latitudes have more ocean, so the effect is more pronounced.

Dr Trenberth said the main reason is storm tracks and strong winds in the upper atmosphere shifting towards the poles and corresponding changes in ocean currents.

He said while warmer seas helped us enjoy long fine summer weather, it is a different matter when weather systems start to get more active in autumn and winter.

Flooding on State Highway 10 at Kāeo during ex-tropical Cyclone Tam in April. Photo: RNZ/Peter de Graaf

"They start to encounter these warmer oceans and suddenly these warmer oceans put more water vapour into the atmosphere which fuels these storms. It means the rainfalls are a lot heavier and we've seen a couple of examples of that over the last two or three weeks, especially with ex-tropical Cyclone Tam."

Dr Phil Sutton, an oceanographer at NIWA, said the latest study fits with what observers have been seeing in oceans around New Zealand for 30 years, and adds to their understanding by looking at both atmospheric changes and oceans.

He said westerly winds have grown stronger and moved to the south, meaning water piles up in the South Pacific's subtropical gyre - a circular system he describes as being like a huge whirlpool - and gets warmer. The core of the changes is happening just northeast of New Zealand, he said.

While the water around Antarctica is often a topic of concern, Dr Sutton said the study confirms that the ocean near us are heating much more rapidly.

"Not to say you shouldn't be highly concerned about even small changes down there, because even small changes could have a huge impact on forming ice, but there's not so much happening down there in fact [compared with New Zealand].

"So New Zealand really is in a very interesting and challenging - interesting scientifically but perhaps concerning environmentally - spot in the world's oceans."

Photo: NIWA / Luke McPake

Dr Sutton has been studying a particularly unusual warming hotspot just south of Chatham Islands, where he said heating has gone "beserk" since 2015 and marine heat waves now persist for more than 200 days of the year.

"We seem to be seeing the first sign of systematic change around New Zealand, where those subtropical waters are pushing into an area where they didn't used to be," he said.

He said his colleagues in fisheries want to know what that means for hoki and orange roughy fisheries in the area, but it was too soon to say.

Dr Sutton was pleased to see that the latest study drew ocean temperatures from a programme of Argo floats, robotic instruments that drift in the ocean, measuring temperature and salinity in the upper 2000 meters.

"At the moment there are about 4100 floats operational around the world and New Zealand has been a big player with the US and Australia in deploying the floats. Around 890 or so of those were deployed by New Zealand, so we have an outsized contribution to maintaining that global array.

"You can sit at your desk and know what the ocean is doing.

"You used to have to get on a ship and ... you'd only know what was happening right where you took the ship."

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