A hole ('polynia') in the sea ice off Antarctica

Return of 'ocean chimney' the size of Tasmania puzzles Antarctic scientists

SMH,

15 October, 2017

A rare hole the size of Tasmania has opened up in the sea ice off Antarctica, enthralling scientists keen to understand its cause and the possible role of climate change in its formation.

Known as the Weddell Sea or Maud Rise Polynya, the ice-free zone appeared in September and has grown to as large as 80,000 square kilometres, according to the University of Toronto.

Polynyas, defined as a stretch of open water surrounded by ice, are frequently found in the Arctic and Antarctica, usually near the coast. They rarely reach the extent now seen in middle of the pack ice.

"Something has changed" to bring the polynya back, Kent Moore, a professor of physics at the University of Toronto, said. "But we not quite sure what that trigger was," he said.

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A much smaller polynya opened up in the same Maud Rise region last year for a couple of weeks. Before that, the previous such event there was in the mid-1970s, with the polynya lasting three years and swelling to as much as five times the current size, Professor Moore said.

Fortuitously, one of 200 biochemical robotic floats released by the Southern Ocean Carbon and Climate Observations and Modelling program has surfaced within the polynya. It is sending back data on temperature, salinity and other measurements – potentially saving a costly mission by a research vessel.

"It's just amazing – fantastic news," Jan Lieser, a sea ice scientist at the University of Tasmania, said of the float's location.

Huge energy release

Normally the sea ice acts as an insulator between the ocean – with waters at freezing point – and the atmosphere, which can be 20 degrees or more below zero.

Water vapour rises off the Antarctic ice sheet as a rare hole opens in the Antarctic winter sea ice. style="box-sizing: border-box; margin: Photo: Jan Lieser, UTAS/ACE CRC

Once opened, the polynya works like "a chimney from the ocean through the sea ice", transferring huge amounts of energy to the atmosphere, Dr Lieser said.

Professor Moore said the polynya was releasing about 800 watts of energy per square metre – equivalent to about 14 60-watt light bulbs blazing away day and night.

The polynya is located east of the Antarctic Peninsula, and even resembles an Orca. Photo: SCAR Atlas

The Maud Rise provides an ideal location for the start of a mid-sea polynya. The rise is a mountain that climbs about 4000 metres to within 1200m of the surface, providing a ramp for relatively warm water flowing along the sea floor.

At its current size, the polynya is big enough to alter the climate locally and potentially a lot further way.

A close-up of the Maud Rise or Weddell Sea Polynya east of the Antarctic Peninsula. Photo: SCAR Atlas

Unusual cloud formation is one clear sign. The heat transfer from the ocean to the atmosphere also cools the surface water, causing it to sink, disturbing sub-sea currents as it falls.

"Whenever you perturb the system, there are obviously consequences," Professor Moore said, adding that the signal of cold water slowly spreading northwards into the Southern Ocean from the 1970s polynya can still be detected decades later.

Still, scientists are unclear what if any links there are between polynyas and global warming.

"It's one of the big unknowns right now," Professor Moore said. "Is this a new normal, or is it just by chance?"  

'Critically important' 

Dr Lieser said researchers are keen to understand polynyas because of their potential to influence the climate in Australia and beyond. Global climate models that project future conditions also need to incorporate them. 

"The Southern Ocean is rather vast – whatever happens down there will have a major effect on other regions," Dr Lieser said.

Given the Southern Ocean's dominant role globally as a sink for both heat and carbon dioxide understanding the processes underway in the region was "critically important", he said. A less efficient sink, for instance, could see more carbon dioxide and heat left in the atmosphere.

One immediate side-effect of polynyas, though, is that they tend to be "really good for wildlife, like an oasis", Professor Moore said.

The warmer than usual conditions and the relative ease for mammals to get to the surface to breathe will likely result in sea leopards and other animals gravitating to the area, he said.

Dynamic ice

The polynya, too, is a reminder of the dynamic nature of Antarctic sea ice.

Unlike the Arctic – an ocean fringed by land – Antarctica is a continent surrounded by ocean. While Arctic ice can be contained by land, winds in the far south can spread the ice, masking its volume.

And so, three years ago, Antarctic ice reached a record large extent at its maximum, at least according to records gathered during the satellite era.

By contrast, Arctic ice has been on fairly clear long-term downward trend both for area covered and ice thickness, in one of the clearest global warming signals.

This year, Antarctic sea ice looks likely to have peaked a maximum cover close to a record low. (See map below showing the record high sea ice extent in blue and this spring's low maximum cover, in red.)

Judging just when this year's sea ice reached its maximum has been a tricky business.

As of October 9, Antarctic sea ice covered 18.064 million square kilometres, which would be close to a record, Dr Lieser said.

Unlike many previous years, however, sea ice extent had risen into spring rather than retreated. (See chart below.)

Sea ice thickness is a more important measure than ice extent, Dr Lieser said.

While decades of submarine activity around the North Pole had provided important data showing the down trend, that information is unavailable for Antarctica.

"We don't have any large-scale reliable estimates of the entire volume of sea ice that's there," he said.