Why our seas scorched: NZ's back-to-back freak marine heatwaves studied

This map shows the sea surface temperature anomalies that were reached - dark red indicates 3C above average - during November 2017. Source / Niwa

Scientists have shed fresh light on two of the most freakish climate events in New Zealand's history – and warned they're likely to become more common as our planet warms.

A new study led by climate scientist Professor Jim Salinger has investigated back-to-back "marine heatwaves" that turned the country's coastal seas into warm baths and drove mass-melting of our glaciers over the summers of 2017-18 and 2018-19.

Salinger said the twin events were the result of an "atmospheric and oceanic stew" including blocking anti-cyclones centred over the Tasman Sea, fewer low pressure systems, and a strongly positive Southern Annular Mode.

When this effect was positive, the massive storms that rage in the "roaring forties" and "furious fifties" latitudes in the southern oceans contract toward Antarctica, instead of in our direction.

The combination of that and a strong La Nina climate system in the tropical Pacific - known to drive more anticyclones east of New Zealand – brought many more anticyclones to our region, while blocking cold swells surging up from the deep south.

But he and his colleagues also pointed to the hand of human-driven climate change, noting that the Tasman had been warmer over the years leading up to the events.

The effects of the heatwaves were dramatic: air temperatures over the country reached between 1.7C and 2.1C above average, while sea surfaces heated up to between 1.2C and 1.9C above average.

Source / Niwa

Over the record-hot summer of 2017-18, some spots off the West Coast soared to between 4C and 6C above normal – and colour-coded sea surface temperature maps showed New Zealand engulfed by red.

Even more dramatic observations included snapper being caught in Doubtful Sound for the first time ever, an early-season bluebottle jellyfish boom and increased sightings of stingrays in Otago Harbour, where temperatures eventually rose to 21C at the end of January.

Algal blooms and kelp forests declined as warm water fish, not usually found so far south, became increasingly prominent.

Source / Supplied

The first heatwave was all the more remarkable in that it peaked twice - once in late December and again in early January - and left seas warm enough to stoke the next event.

When scientists made their end-of-summer stocktake, sea surface temperatures around New Zealand for the period had come in at 1.5C above average - the warmest they'd ever been on records that stretched back 150 years.

It prompted climate agencies here and across the Tasman to jointly issue, for the first time, a historic statement.

The new study also highlighted a rapid melt of seasonal snow on the Southern Alps, causing the loss of an incredible 8.9 cubic km – or 22 per cent of the alps' total ice volume.

A Niwa aerial survey of the Southern Alps revealed the toll wrought by New Zealand's hottest summer. Photo / Dave Allen, Niwa

Niwa's most recent fly-over of the alps' glaciers found they'd struggled to recover from the big melts, which had added to a longer trend that had seen a third of ice wiped out in 40 years.

The heat was similarly borne out in harvests over the two summers.

In most-wine growing regions, the hottest average grape flowering temperatures recorded in more than 20 years had put the season several weeks ahead of normal.

Tim Turvey of Hawke's Bay's Clearview Estate Winery checks on his vines in January 2018. The summer saw hottest average grape flowering temperatures seen in over 20 years. Photo / Duncan Brown

In Marlborough, sauvignon blanc and pinot noir wine grapes had above average berry numbers and bunch mass in 2018, while the cherry growing season was pushed two weeks ahead.

Around the coasts of Southland and Otago, there were "cascading losses" of mussel beds and other seaweeds, the researchers reported.

Lifeguards noticed beaches being crowded much earlier in the season than usual.

Hawke's Bay's Waimarama Beach, on January 14, 2018. The warmer seas and NZ's hottest summer sent crowds flocking to beaches. Photo / Paul Taylor

Salinger told the Herald he'd been surprised by sheer scale of the area affected by the heatwaves, which sprawled over a land and sea area stretching from the central Tasman Sea to the Chatham Islands.

The events were "head and shoulders" above all of the marine heatwaves in New Zealand's recorded history, save for one in the summer of 1934-35, which stood for decades as the country's hottest.

"The 1934/35 event shows such an event is possible without anthropogenic global warming, however such events are becoming more frequent and illustrate what will be a very normal event in few decades if global warming is not mitigated," Salinger said.

Source / Niwa

Ninety per cent of the heat from human-caused global warming going into our oceans and it is likely marine heatwaves will continue to increase.

Recent studies have predicted marine heatwaves are also expected to grow stronger and longer under climate change, with strong influence on temperature and rainfall patterns.

Worldwide over the past century, the frequency of marine heatwaves had increased on average by a third and the length of each grew by 17 per cent.

Combined, it meant the number of marine heatwave days every year had risen by more than half over the period – and there had been a noticeable acceleration of the trend from 1982.

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 started.

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.