Why is Antarctica’s sea ice spreading?

Arctic sea ice maximum hit a record low last year, but down south, Antarctic sea ice was as thick and extensive as ever.

A US team led by NASA’s Jet Propulsion Laboratory’s Son Nghiem suggests why sea ice at the poles is acting differently – a massive “great shield” built early in the growing season protects Antarctic sea ice. The Arctic, though, lacks such a protective boundary.

“Our study provides strong evidence that the behaviour of Antarctic sea ice is entirely consistent with the geophysical characteristics found in the southern polar region, which differ sharply from those present in the Arctic,” said Nghiem.

Sea ice floats on the ocean and extends and contracts seasonally. In Antarctica, sea ice maximum is around September each year.

In recent decades, records for Arctic and Antarctic sea ice have toppled – but in opposite extremes. While the Arctic has seen sea ice maximums shrink, Antarctica enjoyed record sea ice extent in 2012, 2013 and 2014.

So what’s going on?

Climate scientists have put forward a number of theories. One is the hole in the ozone layer above the South Pole may have affected winds high in the atmosphere. Another is water running off Antarctica makes the surrounding ocean less salty, and thus more freezable.

But these don’t adequately explain why Antarctic sea ice is so healthy, even in a warmer climate.

So Nghiem and colleagues mapped sea ice extent during its growing season – June to September – around Antarctica with NASA’s QuikSCAT satellite radar data from 1999 to 2009.

They found early in the growing season, offshore Antarctic winds pushed young, smooth sea ice northwards. At a point, the ice front slowed and the same winds piled more ice atop until it became thick and rough.

This “great shield”, as the researchers called it, protected the sea behind it, like a seawall, to “create ice factories”. Arctic conditions can’t create a shield, so its sea ice is thin and prone to being smashed up by waves.

And the Arctic wind system, the Polar Express, tends to push sea ice into warm currents where it melts.

To understand what it is about Antarctica that lets it build an icy shield, Nghiem and colleagues didn’t just focus on the winds swirling above the frozen continent – they mapped the sea floor around it too.

The shield forms at a sea surface temperature line of -1 °C, where the southern Antarctic Circumpolar Current front separates cold and warm waters.

The Fawn Trough, a deep channel to the east of Antarctica, funnels the current in a tight formation, carving a consistent -1 °C boundary line.

But on the other side of the continent, the current spreads out over the smooth seafloor. This means the -1 °C boundary shifts year on year and accounts for huge variations in sea ice extent off West Antarctica.

The work was published in Remote Sensing of Environment.