Hot: some coastal areas are now in a ‘marine heat wave’

Some of New Zealand’s coastal waters have now warmed to a “marine heat wave” state, an unusual picture that is likely to get even more surprising in the coming months.

Unusually warmer seas are a classic calling card for a strong La Niña system now influencing our climate, but also climate change.

In his latest seasonal outlook, Niwa pointed to the potential for the third marine heat wave in just four years. Now there were early signs that were coming true.

Niwa’s forecaster Ben Noll said data for the month so far showed coastal waters running 1.5 ° C above average in the north of the North Island.

“We are talking about 18 ° C, anywhere up to 20 ° C, and maybe in the Firth of Thames and just offshore in Auckland, we could be reaching 21 ° C,” he said.

“When you mention those numbers to people, they might not think it’s a heat wave, per se, but it should be expressed in how unusual it is for this time of year.

“And we are not even at the peak of our ocean temperature that usually occurs during January and February.”

Nationally, sea temperatures were between 0.7 ° C and 1.5 ° C above average, with cooler waters around the south of the South Island.

“It certainly appears that parts of our coastal waters are in a marine heat wave,” he said.

“In fact, four of our six climate regions are at least 1 ° C above average, which is pretty warm when it comes to the ocean and anomalies.”

In the 30-day period through November 14, areas from the East Cape to Northland were in the 90th percentile or above ocean temperatures.

“The 90th percentile is the threshold at which we would normally say that marine heat wave conditions are occurring in those regions,” he said.

“Further south, while the monthly percentile value has ranged from the 60th to the 80th percentile, conditions are not as abnormal as they are near the top of the country.”

But that could change as New Zealand approaches summer.

“Since our last seven-day ocean temperatures, there has been some development of a hot spot near the west coast of the South Island, which is due to the easterly La Niña winds,” he said.

“You can certainly see that, as of November, ocean temperatures have risen along the western part of the South Island. So even though they are not yet in a marine heat wave, it is worth seeing.”

New Zealand’s recent marine heat waves – the 2017-18 event was the most intense ever recorded – have had dramatic consequences for this part of the country, causing local glaciers to melt at an alarming rate.

Glaciologists have expressed their fear that the frozen wonders may have another big hit this season.

“We are now entering that time of year where we begin to see the correlation between coastal ocean temperatures and land temperatures become quite strong,” Noll said.

“It’s fair to say that these warm coastal seas are certainly part of the seasonal climate that we can expect for the next three months.”

The La Niña system, probably 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 in the west. and south of the South Island.

Noll said La Niña increased the trend of subtropical north winds, a key component of warming in coastal waters and in the Tasman Sea.

Another factor was a crucial indicator of storms in the oceans below New Zealand – the Southern Ring Mode, or SAM – leaning more toward a “positive” state.

“With the positive SAM and the northeast La Niña winds, we get a reduction in the westerly winds that generally help churn the colder waters underground,” he said.

“And then at the bottom, of course, is the impact of climate change. It doesn’t mean that you’re going to have a marine heat wave every year, but it does mean that you’re more likely to experience unusually warm temperatures on land and ocean. .. “

Weather records have shown an obvious trend: 45 consecutive months have passed since New Zealand had a month with below-average temperatures.

Our coastal waters have also warmed at a rate of 0.2 ° C per decade since the early 1980s, and particularly off the west coast of the South Island and east of the Wairarapa coast.

Climate scientist Professor Jim Salinger, who with his colleagues has been watching closely for signs of another marine heat wave, said last month was the warmest October on record, when land and sea temperatures combined. .

“Despite pandemics, global warming continues,” he said, pointing to Napier’s near-record deluge as an example of the extremes New Zealand faces.

A recent study led by Salinger addressed some of the most telling effects of the 2017-18 marine heat wave.

Around the South Island came fish that are normally seen in subtropical waters; the algae that provided habitat for other fish and marine species became extinct; Farmed salmon did not thrive, even surfers could wear shorts instead of wetsuits.

The waters of the Tasman Sea eventually warmed, on average, 3.7 ° C above normal to reach 20.6 ° C, providing a worrying insight into the future.