Why our sea scorched: NZ's incredible marine heatwave

Scientists say we've all just experienced an event that would be considered unusual even 30 years from now, when temperatures could be a degree warmer.

And they don't expect to see anything like the freak "marine heatwave" that turned the Tasman Sea into a warm bath for a long time.

It fired our record-hot summer, melted ice caps and lured swarms of jellyfish to our shores.

And although summer is long behind us, the marine heatwave and its effects on our weather linger on.

Before it finally vanishes over the next three months, it will deliver us a parting gift - a strong hand in above-average temperatures for most of winter.

It's already been the warmest start to a New Zealand calendar year.

That the marine heatwave happened in the first place was due to a complex but remarkable mix of circumstances.

Marine heatwaves as we know them are created by either stronger-than-normal warm ocean currents - or forced by a dramatic set-up in our atmosphere.

This was a case of the latter.

It was born in November from light winds and easterlies, and more anticyclones than normal to the west and east of the South Island, which effectively dampened down the usual westerly wind swells coming out of the oceans to our south.

Junior-Kauapa Vaha, 8, throws water over Luta Tanei, 7, during a hot Christmas Day at Auckland's Mission Bay beach. Photo / Brett Phibbs

In the background was the rare pairing of a strong La Nina climate system in the tropical Pacific - known to drive more anticyclones east of New Zealand - with a positive phase of what's called a Southern Annular Mode, or SAM.

When this effect is 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 those two drivers brought many more anticyclones to the south Tasman Sea and east of the South Island and blocked cold swells surging up from the deep south.

This graphic shows the sea surface temperature anomalies that were observed on January. Image / Supplied

Then, it was only a matter of sunny conditions warming up the top layer of the sea.

Sea-surface temperatures climbed sharply in the southern Tasman Sea in November, reaching 2C or more above average over a sprawling area between here and Tasmania.

They only warmed more in December, hitting 2.5C and 4C above average through much of the month.

Some spots off the West Coast soared to between 4C and 6C above normal.

Colour-coded sea surface temperature maps showed New Zealand engulfed by red.

The marine heatwave as observed in late November. Image / Ben Noll, NIWA

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.

Tim Turvey, owner of Hawke's Bay's Clearview Estate Winery, checks the vineyards on January 24. The marine heatwave had a dramatic effect on wine-growing. Photo / Duncan Brown

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

Climate scientist Dr Jim Salinger, who plans to carry out a major study into the event, said swimmers and surfers also noted the unusual warmth of the waters around the South Island - a region normally noted for its freezing surf.

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

On land, hot temperatures in the South Island had caused a massive melt-off, with a marked loss of permanent snow and ice in the Southern Alps.

The marine heatwave was a big factor in the mass melt-off. Photo / Dave Allen/Niwa

Scientists who flew over the glaciers last month were stunned at how sad and dirty they appeared.

Levels at the South Island's hydro lakes were also notably low and the postcard hills of the Mackenzie country were bare of snow very early, melted away in the blistering sun.

Salinger noted how the first prediction of the marine heatwave was made in Niwa's December climate outlook, by which time it had already bedded in.

Could we have seen it coming earlier?

"With good climate models tuned to the region, as they have at [Australia's] Bureau of Meteorology, we could have," he said.

"However, I don't believe these climate forecasting modelling capabilities have been achieved yet in New Zealand for marine heatwaves."

Scientists surveying the South Island's glaciers last month saw them looking sad and dirty after the record-hot summer. Photo / Dave Allen/Niwa

Even when it was recognised, the heatwave was still much more powerful than meteorologists could have imagined.

Salinger put that down to the incredible persistence of the positive SAM and La Nina throughout most of summer.

"This resulted in very persistent anticyclones to the southeast of the South Island in December extending into the Tasman Sea, and in January very persistent anticyclones to the southeast of the South Island extending to the south, and northeasterlies," he said.

"Any cold outbreaks and westerlies from the southern oceans did not get a look-in."

The heatwave also managed to peak twice - once in late December and again in early January.

Source / Niwa

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.

Breaking the summer into weeks turned up figures yet more startling - some saw sea surface temperatures 3C above average - and Salinger expected there was "certainly a touch" of climate change in the mix.

Ultimately, the marine heatwave was one of the shining highlights of a summer that will be better remembered for the fact it was our hottest in recorded history, coming in a scorching 2.1C above average, packing our hottest-ever January, and eclipsing even the yardstick summer of 1934-35.

"The 1934/35 event was 1.9C above average - and no summer had come near it until 2017/18," Salinger said.

Source / Niwa

"This time it was enhanced because of global warming."

But, he added, the chances of another positive and extended SAM and a La Nina event hitting us with the same one-two punch remained small.

"Even with mid-range global warming by 2050, the chances are low."

Out on the ocean, its effects lingered on.

There was still enough hot water in the bath to keep sea surface temperatures east of the South Island at 2C above average, and 1C above average elsewhere around New Zealand.

Salinger said all models showed that pattern slowly fizzling out across the next three months.

"So expect above-average sea surface temperatures to stay around until winter."