Another marine heatwave? NZ's seas are warming fast

Scientists are not ruling out another marine heatwave engulfing New Zealand and further melting glaciers this summer, as seas in some areas warm past 20C.

Meteorologists have been watching some dramatic spikes in sea surface temperatures (SSTs) over recent weeks, especially around the north of the country.

At Omokoroa, north of Tauranga, water temperatures soared from about 15.5C halfway through November to nearly 25.5C by the month's end.

Over the same hot, settled week that surge happened, several land temperature records were broken around the North Island.

Throughout last month, SSTs climbed to reach 0.78C above normal off the east coast of the South Island, and 0.39C above normal around the north of the country.

"A stormier start to December has meant coastal waters have had a lot more mixing and sea temperatures actually dropped slightly for a while - but are on the rise again for many parts of the country," said Niwa meteorologist Nava Fedaeff, adding that there might be another slight dip in the week ahead.

In September this year, it proved the first time in more than 30 months that seas were cooler than normal.

But by October, SSTs were back to near average; a month later, they were returning toward "above average".

She couldn't discount the chance of another marine heatwave – which would make it the third in three years.

These dramatic events could drive further melting of glaciers, push warm water fish south and fuel soaring temperatures on land.

"It's something we are watching very closely given how quickly sea temperatures have bounced back since spring."

Fedaeff said it was that cooler spring that had set SSTs apart from those at this point in 2017 and 2018: in both years, waters in the Tasman Sea had stayed warm over the winters before.

Over the summer of 2017/18, particularly – New Zealand's hottest in history – waters had been primed by a La Niña climate system, whose influence lingered on all through the next year.

"That gave us a head start last summer and persistent high pressure in January and February, with frequent bouts of hot air masses from Australia contributing to last year's marine heatwave."

The drivers that proved the difference this year was a positive phase of something called the Indian Ocean Dipole – heavily influencing ocean temperatures around Australasia – and a negative phase of another indicator called the Southern Annular Mode, which reflected storminess deep below New Zealand, toward Antarctica.

But Fedaeff said both of these Tasman-cooling effects were weakening – and therefore, their influence was unlikely to extend much beyond December.

If the warming trend continued, she expected it would have some influence on our air temperatures during the summer season - either by modifying any cooler air attempting to arrive from the southwest, or enhancing warmth around the country.

"Warmer seas can also mean that there is more 'fuel' for incoming storms," she said.

"Previous work has shown that there is a strong correlation between sea temperatures and the temperatures in the Southern Alps."

Notably, marine heatwaves slashed the potential for any of the snow that fell in the previous winter period to last through summer and into next year – and over the 2017/18, scientists observed the biggest melt in four decades of records.

"There are of course a myriad of other impacts associated with marine heatwaves - hence why we are watching the seas very closely."

Climate change is predicted to make marine heatwaves more frequent - and more intense.