The 'hot blob' off the New Zealand coast: All you need to know

The patch, or blob, of hot water is a bit too far off the New Zealand coast for Kiwis to feel its benefits, but it is "extremely warm", experts say. Image / ClimateReanalyzer.org

It's the summer marine heatwave attracting huge attention - a massive blob of hot water off the eastern coast of New Zealand.

Right now the southern reaches of the Pacific Ocean is one of the warmest sea spots on the planet with temperatures of up to 20C appearing as a bright red blob on satellite images.

Professor James Renwick, a weather and climate researcher at Victoria University, has a simple explanation for the phenomenon: "It's just a patch of water that's had a lot of sunny skies and not much wind".

In fact, a very big patch of water measuring tens of thousands of square kilometres where the water is 4C above the average temperature of 10c to 15C on a similar latitude to Wellington in the Pacific Ocean.

The central hot spot is about the size of the North Island(114,000sq km) or the South Island(150,000sq km). The wider area is larger than both islands combined.

"Sea temperatures don't actually vary too much and a degree, plus or minus, is quite a big deal and this area is probably four degrees or more than that above average and that's pretty huge.

"Right in the centre of the "blob" it's likely to be more than six degrees warmer than average.

"It's extremely warm water in terms of differences from average, it's got to be one of the warmest spots on the planet at the moment," said Renwick.

At 20C, the blob in the southern Pacific is getting up there with temperatures in the Tropics that start in the 20s and reach 30C.

The coldest seas are in Antarctica and the Arctic Ocean where water freezes at -2C because of salt content.

HOTBLOB

Renwick said the waters out to the east of the country had been experiencing "quite high pressures, sunny skies, light winds, so the surface of the ocean would warm up quite quickly".

"But if the winds are strong then it gets all stirred up.

"If it's not being stirred enough, the warming from the sun and so on will just be absorbed at the surface of the ocean where you get this quite thin layer of very warm water, so what I'm wondering is, how deep is this very warm blob?"

Renwick said it was likely the shallow layer of warm water, possibly up to 50m below the surface, would likely fade and then dissipate as it heads towards South America.

This will happen as the wind picks up and the layer of warm water mixes with cooler water over the next few weeks.

How far it travels towards South America, Renwick could not say, but if it did not break down over the next month and lasted for a couple of months "it may get reasonably close" but doubted it would reach the coast.

The patch, or blob, of hot water is a bit too far off the New Zealand coast for Kiwis to feel its benefits, but it is "extremely warm", experts say. Image / ClimateReanalyzer.org

Renwick said the blob was not a sign of global warming, just what happens in the ocean. A similar blob occurred at the Bering Sea in September.

An image, taken on Monday by the Climate Reanalyzer website, comes after the Herald reported earlier this month that scientists weren't ruling out another marine heatwave which will further melt glaciers over summer.

Throughout November, some sea temperatures grew up to 10 degrees in a matter of weeks while land temperature records were also breaking.

"A stormier start to December has meant coastal waters have had a lot more mixing and sea temperatures actually dropped slightly for a while - but are on the rise again for many parts of the country," said Niwa meteorologist Nava Fedaeff, adding that there might be another slight dip in the week ahead.

In September this year, it proved the first time in more than 30 months that seas were cooler than normal.