Marine heatwave: Will the Tasman Sea temperature rise again?

As summer approaches, scientists are trying to predict whether sea temperatures around New Zealand will reach the record levels they did last year. 

Scientists at the National Institute of Water and Atmospheric Research (NIWA) are keeping an eye on sea temperatures, as this time last year a "marine heatwave" event was "well and truly underway". 

But NIWA meteorologist Ben Noll says climate patterns have been more variable this year, telling Newshub on Monday that as it stands right now, "last year was in a different league compared to this year".

"It is warmer than average out there around New Zealand and in the Tasman Sea but compared to this time last year it's a far cry from that point," he said. The marine heatwave developed in November 2017, and peaked in December 2017 to January 2018.

New Zealand recorded its hottest January on record this year, and a Tasman Sea marine heatwave is said to have played a large part. The last one happened in 1934-35, but this period has been warmer by 0.4degC because of climate change. 

"Typically, we're used to experiencing heatwaves on land - but in this case it was happening in the ocean, which can certainly happen with the right characteristics of the climate," said Mr Noll. 

What causes them? Lots of sunshine, he says. "The sun can heat up the surface of the ocean and the more sunny days you have the more heating can occur."

"A second factor is light winds. It means that the ocean can be quite still and not very choppy. When the ocean is not very choppy it means that the cooler water that sits beneath the ocean surface can't get churned up to the top."

The long-term trend is that sea temperatures will continue to warm up over time and that "fits in the mould of climate change that we're experiencing warmer oceans on earth and I think the Tasman Sea is part of that grander scheme," said Mr Noll. 

"That doesn't mean that it can't get cold for a month, but the long-term trend is certainly for warming seas, not only in the Tasman, but around the world."

It's not expected to get to an extreme level that we saw last year anytime soon. However, Mr Noll expects there to be another in a couple of years or even decades. 

"The conditions that came together last year to produce the marine heatwave were rather spectacular and hadn't quite occurred to that magnitude in many decades. In fact, one of our scientists here at NIWA, Dr Brett Mullan, compared it to something that happened in 1934-1935.

"The perfect storm of conditions for this heat wave in the ocean occurred last year and it certainly doesn't look like that's on the cards here for the start of this summer."

Do marine heatwaves relate to El Niño?


El Niño and marine heatwaves are "two very different things", says Mr Noll. 

"El Niño takes place in the tropical Pacific and near the equator so many thousands of kilometres away from us in New Zealand.

"Marine heatwaves and El Niño are generally unrelated, however El Niño can bring more wind that blows from the southwest or west and that can lead to a cooler Tasman Sea."

He said it's unusual to have a warm Tasman Sea during such a weather event. 

"They usually bring about a cooler Tasman Sea, but in this case it is above average so maybe that speaks to the flavour of this El Niño being a bit unusual."

Mr Noll said last month there was now an 80 percent chance an El Niño will happen this summer, which he said could see drier-than-normal conditions.