Winter winds blow Arctic sea ice into melt zone

Old Arctic ice, known as “perennial” ice, is ice that has survived at least one summer. It is thicker than new ice, is less prone to melting, and helps keep Earth cool in summer by reflecting sunlight. It is also a vital part of the habitat of animals such as polar bears and seals. However, the coverage of perennial Arctic ice has reduced by around 50% since 1984 and is currently at its lowest extent on record for this time of year.

The new study, published in the journal Nature Communications Earth and Environment, found that winds blowing persistently clockwise for most of the winter drove 8% of the Arctic’s entire perennial ice store into the Beaufort Sea, a southerly region where two thirds of ice melts in the summer.

This resulted in almost a quarter (23.5%) of the Arctic’s perennial ice sequestered in melt-prone regions outside the central Arctic – the largest fraction since records began in 1989.

Robbie Mallett (UCL Earth Sciences), a London Natural Environment Research Council (NERC) DTP funded PhD student who led the study, said: “In mid-February we saw that a strong weather system was literally spinning the Arctic sea ice around. When we looked closer we saw that the Arctic’s older ice was drifting out of the ‘survival zone’ - the shrinking area where ice can still survive the summer melt season.”

Co-author Professor Julienne Stroeve (UCL Earth Sciences and the University of Colorado Boulder) said: “Since perennial ice tends to be thicker ice that is more resistant to melting out during summer, the loss of this ice removes the ocean’s reflective ice cover, allowing the ocean to absorb more heat and light, which in turn melts more ice and warms the planet.”

The persistent clockwise winds that spun the Arctic sea ice around were linked to a weakening of the polar vortex – a circulation of cold winds high up in the Arctic atmosphere. This weakening caused cold weather to move southward, causing UK temperatures to fall to their lowest level since 1995. In Texas, extremely cold weather paralysed the power grid, leaving four million people without power. The Arctic, meanwhile, had very high air pressure, warmer temperatures and unprecedented anticyclonic (clockwise) winds blowing more persistently than ever recorded.

In the study, researchers analysed weather data alongside satellite-derived maps showing the thickness of sea ice, how it was moving around, and the coverage of perennial versus first-year ice, over December, January and February.

To characterise the winds over the sea ice the researchers analysed weather data produced by a supercomputer operated by the European Centre for Medium Range Weather Forecast in the UK.

To investigate the response of the sea ice, the researchers looked at maps of perennial ice produced by the National Snow and Ice Data Center in Boulder, Colorado, and derived from satellite-based microwave radiometers. (These instruments distinguish first-year ice from perennial ice and open water by measuring the microwave energy emitted from their surfaces.)

Using data generated by the European Space Agency’s CryoSat-2 mission, they found that the movement of older, thicker ice to melt-prone regions contributed to a record low in the thickness of ice remaining in the Arctic Ocean “survival zone” at the end of winter – the region where ice normally survives the summer melt season.

As well as being thinner, perennial ice is accounting for a highly diminished proportion of the Arctic region – its coverage was low at the start of the year, and is now at the lowest extent ever seen for this time of year, beating a previous record set in 2017.

“Last winter things started badly, and then they got worse,” said Mallett. “We saw a record-late start to the winter growth season, from which the sea ice never really recovered. To add to that, we now have a record-low and precariously positioned perennial ice cover in the Arctic’s hottest months. If this ice melts before September, then it’s gone.” 

Professor Stroeve found in a previous study that when the perennial ice is blown into the southern Beaufort Sea, much of that ice will melt out the following summer.

Links

• Robbie Mallett’s academic profile
• Professor Julienne Stroeve’s academic profile
• UCL Earth Sciences
• UCL Faculty of Maths & Physical Sciences
• University of Colorado Boulder
• European Centre for Medium Range Weather Forecast
• National Snow and Ice Data Center

Image

• Iceberg, Pixabay

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