New Zealand's waters have warmed dramatically over recent weeks – but a scientist says a key climate driver would need to "flip" to send seas into another mountain-melting "marine heatwave".

Sea surface temperatures have reached above 20C in some regions around the country; and throughout November, waters off the South Island's east coast hit 0.78C above normal.

Niwa wasn't ruling out the emergence of the third marine heatwave in three years – but has pointed out that more factors than just warmer seas were needed to create one.

Marine heatwaves have been known to melt glaciers, send tropical fish into colder climes and turn our beaches into warm baths.

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They're also potentially devastating for marine ecosystems, and are expected to grow stronger, longer and more frequent under climate change.

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 came amid La Nina conditions and 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 growth in orchards and vineyards.

Source / Niwa
Source / Niwa

Another marine heatwave followed the next summer, with oceans having not recovered from the first event.

Climate scientist Professor Jim Salinger said the 2017/18 went down as the most intense marine heatwave on record, followed by an event in 1934/35, and the 2018/19 episode.

But this summer, he said the atmospheric conditions needed to set up a heatwave weren't locked in place – at least yet.

Specifically, an indicator called the Southern Annular Mode – or SAM – needed to switch from a negative phase to a positive one.

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The SAM measured the north–south movement of the westerly wind belt that circled Antarctica, dominating the middle to higher latitudes of the southern hemisphere.

It was the changing position of that westerly wind belt which influenced the strength and position of cold fronts and mid-latitude storm systems over New Zealand.

A positive SAM had meant there were weaker westerly winds than normal over the South Island with higher pressures - and less cold fronts crossing New Zealand with bursts of cold air.

In a positive phase, it also blocked highs to the east of the country, and sometimes bulging back over New Zealand, with mild northerly airflows across New Zealand.

But currently – and at least for the short term – the SAM was in a negative phase, reflecting increased westerlies and more unsettled weather over New Zealand, with eased windiness and storm activity over the southern oceans.

"When the SAM is negative – and it's predicted to stay negative – it doesn't encourage those big anticyclones that will calm the seas down, and stop those troughs, which we're currently getting," Salinger said.

Source / Niwa
Source / Niwa

Those "blocking" anticyclones also played a part in allowing the Eastern Australian Current to push more warm water down into the Tasman Sea, pushing temperatures up further.

"So put it this way: the ocean is conditioned, but we still don't have the right atmospheric conditions we need to get a heatwave out of it," he said.

"I'd add the Southern Annular Mode is something that can change pretty quickly: but it's been very negative during this current period."

There was much about marine heatwaves that scientists still had to understand – and they were tough to predict because there were many factors influencing how and when they kicked off.

Niwa scientists have been combining models with observations from satellites and ocean drones to gain some new insights - including a clear link between changes in heat content in the Tasman Sea and the size and intensity of marine heatwaves over years and decades.