Some of New Zealand's coastal waters have now warmed to a "marine heatwave" state - an unusual picture likely to grow even more striking over coming months. Image / Niwa
Some of New Zealand's coastal waters have now warmed to a "marine heatwave" state - an unusual picture likely to grow even more striking over coming months. Image / Niwa
Some of New Zealand's coastal waters have now warmed to a "marine heatwave" state - an unusual picture likely to grow even more striking over coming months.
The unusually warmer seas are a classic calling card of a strong La Nina system that's now influencing our weather - but also of climate change.
In its latest seasonal outlook, Niwa flagged the potential for the third marine heatwave in only four years. There were now early signs that was coming to bear.
Niwa forecaster Ben Noll said data for the month so far showed coastal waters were running 1.5C above average around the north of the North Island.
"We're talking about from 18C, anywhere up to 20C - and maybe in the Firth of Thames and near shore in Auckland, we could be reaching 21C," he said.
"When you mention those numbers to people, they may not think that's a heatwave, per se - but it needs to be couched in how unusual that is for this time of year.
Source / Niwa
"And we're not even at the peak of our ocean temperatures that typically occurs over January and February."
Nationally, sea temperatures were tracking at between 0.7C and 1.5C above average, with waters coolest around the south of the South Island.
"It certainly looks like parts of our coastal waters are in a marine heatwave," he said.
"In fact, four out of our six climate regions are at least 1C above average - which is quite warm when it comes to the ocean and anomalies."
In the 30-day period up to November 14, areas from the East Cape through to Northland were sitting at or above the 90th percentile for ocean temperatures.
"The 90th percentile is the threshold where we'd usually say, marine heatwave conditions are occurring in those regions," he said.
"Further south, while the monthly percentile value has ranged anywhere from the 60th to the 80th percentile, conditions are not quite as anomalous as they are near the top of the country."
But that could change as New Zealand moved closer to summer.
"From our latest seven-day ocean temperatures, there has been a bit of a development of a hot spot near the West Coast of the South Island, which owes to La Nina's easterly winds," he said.
"You can certainly see that, through November, ocean temperatures have increased all along the western part of the South Island. So, while they're not in a marine heatwave yet, it's worth watching."
People take to the water at Auckland's Mission Bay on Saturday. Coastal water temperatures in northern New Zealand have already been running 1.5C above normal. Photo / Sylvie Whinray
New Zealand's recent marine heatwaves - the event of 2017-18 was the most intense ever recorded - have had dramatic consequences for this part of the country, driving local glacier melt at alarming rates.
Glaciologists have sounded fears the icy wonders could be in for another big hit this season.
"We're now getting into that time of year when we start to see the correlation between coastal ocean temperatures and land temperatures becoming quite strong," Noll said.
"It's fair to say these warm coastal seas are certainly part of the seasonal climate we can expect over the next three months."
The La Nina system - likely to prove the strongest in a decade - is predicted to bring above average temperatures everywhere, but with wetter conditions in the north and east of the North Island, and drier weather around the west and south of the South Island.
Noll said La Nina increased the tendency for subtropical northerly winds - a key component of warming in coastal waters and in the Tasman Sea.
Another factor was a crucial indicator of storminess in the oceans below New Zealand - the Southern Annular Mode, or SAM - leaning more toward a "positive" state.
"With the positive SAM, and La Nina north-easterlies, we get a reduction in westerly winds that usually help churn up the cooler waters at the subsurface," he said.
"And then in the background, of course, is the impact of climate change. It doesn't mean you're going to have a marine heatwave every year, but it means you're more likely to experience unusually warm land and ocean temperatures."
Weather records have shown an obvious trend: it's now been 45 consecutive months since New Zealand had a month with below average temperatures.
Our coastal waters have also warmed at a rate of 0.2C per decade since the start of the 1980s - and notably off the South Island's west coast, and east of Wairarapa's coast.
Climate scientist Professor Jim Salinger, who with colleagues has been closely watching signs of another marine heatwave, said last month was the warmest October on record, when land and sea temperatures were combined.
"Despite pandemics, climate warming continues," he said, pointing to Napier's near-record deluge as an example of the extremes New Zealand faces.
New Zealand's most severe marine heatwave as observed in late November, 2017. Image / Ben Noll, Niwa
A recent study led by Salinger touched on some of the most eye-opening effects of the 2017-18 marine heatwave.
Around the South Island, fish normally seen in sub-tropical waters arrived; kelp that provided habitat for other fish and marine species died off; farmed salmon failed to thrive – even surfers could wear board shorts instead of wetsuits.
Waters of the Tasman Sea ultimately warmed, on average, by 3.7C above normal to reach 20.6C - providing a worrying glimpse of the future.
