They melt mountains, send tropical fish into colder climes and turn our beaches into warm baths.

But precisely what triggers marine heatwaves – a phenomenon that's helped make New Zealand's past two summers among the hottest on record – has been tough for scientists to nail down.

Now, researchers have been able to draw a new link between their formation in our oceans, and the length of time they keep our seas warmer than normal.

Marine heatwaves are potentially devastating for marine ecosystems but are expected to become more frequent under climate change.

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Recent studies have shown they also have a strong influence on climate, temperature and rainfall patterns.

New Zealand's summer of 2017-18 – our hottest on record – included a freak marine heatwave that pushed sea surface temperatures to 1.5C above average, and as high as 6C above normal in some spots off the West Coast.

While its effects made for balmy surf at our favourite beaches, it also drove the biggest melt ever seen on the Southern Alps, pushed warm water fish south and had a big effect on growing conditions in orchards and vineyards.

Niwa scientist Dr Erik Behrens said these events were difficult to predict because there were many factors influencing how and when they kicked off.

But, by combining models with observations from satellites and ocean drones, he was able to show a relationship between changes in heat content in the Tasman Sea and the size and intensity of marine heatwaves over years and decades.

"Measuring ocean heat content is like measuring fever but in the ocean and tells you a lot about its wellbeing," said Behrens, whose findings have just been published in the journal Frontiers in Marine Science.

A marine heatwave which took hold last summer drove another big melt on the Southern Alps, pictured here in March. Photo / Drew Lorrey, Niwa
A marine heatwave which took hold last summer drove another big melt on the Southern Alps, pictured here in March. Photo / Drew Lorrey, Niwa

"If the ocean is already warm, the likelihood for marine heatwaves is enhanced because less surface heating is required to drive temperature extremes."

His study showed heat fluctuations in the Tasman Sea were dominated by more warm water being transported from the subtropics by two major ocean currents - the East Australian Current and Tasman Front - with the heat content acting setting up the development of marine heatwaves.

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"The flow of warm water from the subtropics varies a lot, which results in warmer and colder years," Behrens said.

"But on average the Tasman Sea is a region of oceanic heat convergence, meaning that average surface heat fluxes are directed toward the atmosphere, balancing the oceanic heat convergence."

On average, just over two thirds of the ocean heat transported from the subtropics into the Tasman Sea warmed the atmosphere.

He explained there were at least two different mechanisms affecting the occurrence of marine heatwaves in the Tasman Sea: heat content which was persistent and enabled long-term predictability, and winds and surface heating which were harder to predict, given they varied on shorter time scales.

"Periods with enhanced heat content or when ocean heat content increases rapidly, increase the likelihood of marine heatwaves developing," Behrens said.

"However, if winds are strong during the same time then the marine heat waves are milder."

Marine heatwave conditions continued in the Tasman Sea throughout last month.

Niwa forecasters expect these warmer-than-average seas may lead to milder and shorter cold spells over winter.