Scientists have been alarmed to discover an intriguing process capable of melting massive floating Antarctic glacier from above - even during the frozen continent's dark and wild winters.

And the winds at the centre of the surprise finding were just the same type that help keep Canterbury dry over summer.

In New Zealand, what are called Foehn winds occur when westerlies arrive at the South Island's West Coast and have to climb over the Southern Alps.

As they go, they cool and lose water before descending down the eastern side much drier and warmer, and eventually helping brown the plains of Canterbury.

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Now scientists have revealed that a similar Foehn wind, climbing over the mountains of the Antarctic Peninsula, can warm so intensively that they reach positive temperatures even during the frozen continent's dark and hostile winters - a result that's surprised them.

Concerningly, this effect was observed at the 3250sq km Larsen C ice sheet, a large floating glacier in the north of Antarctica, where a large iceberg broke off just last July.

Over winter in Antarctica, conditions are pitch-black and freezing cold for months on end, and temperatures in the centre of the continent can drop to minus 80C.

On the coast, however, the winter is usually slightly milder by Antarctic measures, at around minus 25C.

Foehn winds are known to help keep Canterbury dry over summer. Photo / File
Foehn winds are known to help keep Canterbury dry over summer. Photo / File

When the mercury rises above zero, snow begins to melt, causing several meltwater lakes to accumulate on top of the underlying glacier.

These lakes could be 50m wide, up to a kilometre in length and one or to two metres deep.

'It really was that warm'

Glaciologist Dr Peter Kuipers Munneke, of Utrecht University, and colleagues took a closer look at the lakes by comparing new satellite data with older imagery and measurements on the ground to discover about 20 per cent of the observed melting was happening during winter.

"We hadn't expected it to melt so much there in the winter, because it's so dark there, and the sun provides absolutely no heat," he said.

"Four years ago, we installed a weather station there to study why so much snow melts in the area.

"Unexpectedly, it's due to the melting in the winter, which appears to be caused by the warm wind."

The glacier researchers' basecamp on the location of summer meltwater lakes. Photo / Nick Gillett
The glacier researchers' basecamp on the location of summer meltwater lakes. Photo / Nick Gillett

Around once a week, the Foehn winds blow down from the mountains to the west of the ice sheet, raising the temperature by 15C to 20C in just a few hours.

"All of the winter heat comes from the Foehn wind, as there is no other heat source this period of year," he said.

"During a strong Foehn, so much snow can melt that it forms huge lakes on the surface of the ice.

"We had known about these lakes during the summertime, but apparently 20 to 25 per cent of the meltwater from the past few years actually occurs in the winter instead."

Munneke was first told of the curiously high temperatures in 2016 by a colleague who questioned a reading from a weather station at the site: it strangely showed 8C.

"At first, I thought that there was something wrong with the instrument, or that it was a value that needed to be corrected for other weather influences that make it seem warmer than it is.

"But that wasn't the case. It really was that warm."

Munneke said the secret was ultimately revealed thanks to new high-resolution satellite images from the European Space Agency, which, compared with previous images, was like trading "camera on your first mobile phone for the latest iPhone 8".

The satellite image of 27 May 2016 shows parallel winter meltwater lakes on the Larsen C icesheet. Photo / ESA
The satellite image of 27 May 2016 shows parallel winter meltwater lakes on the Larsen C icesheet. Photo / ESA

These revealed that the meltwater re-froze over the course of the winter.

"Both of the times, the lakes developed in May, the start of the Antarctic winter. Later in the season, the water re-froze, creating slabs of ice over those locations.

"Consequently, the meltwater doesn't flow into the sea, so it doesn't contribute to rising sea levels either."

However, he said, the discovery may have consequences in the future.

"Over the past few decades, large floating glaciers have broken off from the Antarctic mainland.

"That was in some cases caused by the fact that large meltwater lakes had made some of the ice sheets unstable.

"We may be observing a process that might create meltwater lakes over a much larger area in the future."

The weather station operating at the Larsen C ice sheet during summer. Photo / David Ashmore
The weather station operating at the Larsen C ice sheet during summer. Photo / David Ashmore

Munneke expected there would be many more winter melt days as time passes.

"Not only because global temperatures are rising, but also because a warmer world means a stronger westerly wind in the Southern Hemisphere.

"And that increases the Foehn winds that cause winter melting in Antarctica."

The results ultimately offered the first insight of winter melt on Antarctica.

"Thanks to older satellite images, we already had a good idea of the number of winter melt days since 2000," he said.

"Only now, by combining the new images and exact temperatures, we understand the consequences."

But he added it was unclear of whether this was happening now more than in the past.

"This is an initial study, that we can use to compare the volume of winter melting in the future."

The bigger picture

Professor Nancy Bertler, of Victoria University's Antarctic Research Centre, said the wider West Antarctic Ice Sheet was especially vulnerable to lose mass, as large regions of its ice rested on ground that lay hundreds to thousands of metres below sea level.

The ozone hole and global warming have influenced westerly winds, driving them closer to Antarctica and speeding them up.

In turn, these winds drove ocean currents of warm salty water under the cold underbelly of the ice sheet.

Since observations started a few decades ago, these warm currents led to rapid thinning of ice shelves - the floating tongues of the ice sheet - and shrinking of the ice sheet in this region.

While the warm ocean waters were the principle drivers of this alarming mechanism, scientists recently learned that atmospheric warming had an important role as well.

"As ice shelves respond to the warmer waters from below, the warming air above can provide a fatal blow," Bertler said.

That was particularly visible in 2002, when melt puddles developed on the large Larsen B ice shelf, leading to its catastrophic collapse over the course of a few short weeks.

"While the collapse of the ice shelf doesn't raise sea level, the ice resting on land behind has accelerated into the ocean, contributing to the rising tide, and they do so to this day," Bertler said.

A 180km rift in Antarctica's Larsen C ice shelf drew international attention last year. Photo / Nasa
A 180km rift in Antarctica's Larsen C ice shelf drew international attention last year. Photo / Nasa

"Since then, we saw more examples of this process in Antarctica, a mechanism which is now starting to be captured also in computer models that help us to predict future contributions to global sea level rise."

Some West Antarctic ice shelves had thinned by almost 20 per cent in the last 20 years and overall, Bertler and colleagues observed a 10-fold increase in Antarctica's contribution to global sea level rise.

She said the new study was particularly interesting as it effectively identified a new and highly surprising mechanism to pool melt waters on the surface of Antarctica's vulnerable ice margins.

"It seems his team was able to shed light on a peculiar secret so far well hidden in the long months of the Antarctic night."