What new underwater images reveal about Antarctic sea ice

Wednesday, November 26, 2014, 11:10 AM - Mapping sea ice is usually a job for orbiting satellites, but in an effort to get the the full picture, scientists are turning to a submersible robot to give them the view from underneath the ice too. What they found shows that the ice surrounding Antarctic is thicker and more deformed than they previously thought, which may help them to refine models of ice growth and climate change.

When assessing how much sea ice is floating around in Earth's oceans, in order to understand its full impact on our climate system, we need to know the total amount - not only how much of the ocean surface it covers, but also how thick the ice is to get the full ice volume. Although ice extent can grow and shrink by quite a bit from season to season, and the amount of melting and growth can vary from year to year as well, knowing the total volume gives us a better idea of how the ice is responding to climate change.

For example, after the record melt of Arctic sea ice in the northern summer of 2012, the melts in 2013 and 2014 were more in-line with the years prior to 2012 (and very close to what was seen in 2009). Although some have claimed this to be a 'recovery' of the Arctic, the 2012 melt was due to a combination of the warming trend in the climate and a specific worst-case sequence of weather events during that summer. Since that sequence of weather events didn't repeat in the years after, the ice extents didn't reach as low. However, the ice that grew back after 2012 was far thinner than was there before that melt, so the overall volume of ice - the total amount, rather than just how much ocean it covers - is still at a record low for the Arctic.

At the other end of the world, the ice surrounding Antarctica has been growing slightly (~1.5 per cent per decade, compared to the ~11 per cent per decade loss of Arctic sea ice), and it's mainly been assumed that the sea ice there is a relatively thin sheet, perhaps about 10 metres thick.

Satellites can gather thickness data, but satellites have a hard time differentiating between ice and the snow covering that ice. Drilling through the ice can provide a better idea, but there's always been gaps in these mapping efforts, since ships need to avoid some areas of ice floe, due to the danger of the ship becoming stuck or damaged.

Putting a submersible into the water beneath the ice can allow researchers to get a better look at what's there, but the trick is to have a submersible that's stable enough in its maneuvers to take reliable measurements.

Designed, built and operated by Woods Hole Oceanographic Institution (WHOI), SeaBED is an Autonomous Underwater Vehicle (AUV) specifically designed to dive underneath sea ice, and rather than just take observations, it maps the underside of it using upward-facing sonar.

"SeaBED's maneuverability and stability made it ideal for this application where we were doing detailed floe-scale mapping and deploying, as well as recovering in close-packed ice conditions," said Hanumant Singh, the Woods Hall scientist who's lab built SeaBED, according to a National Science Foundation press release. "It would have been tough to do many of the missions we did, especially under the conditions we encountered, with some of the larger vehicles."

By setting SeaBED on a back-and-forth 'lawnmower' pattern for its mapping survey, the team - consisting of researchers from the Institute of Antarctic and Marine Science and Antarctic Climate and Ecosystem Cooperative Research Centre in Australia, the Woods Hole Oceanographic Institution in the United States and the British Antarctic Survey - was able to compile maps covering about 500,000 square metres, in three different regions around the Antarctic Peninsula. Their findings showed that the ice in these regions, which was inaccessible in the past, is much thicker than previously thought - up to 16 metres thick in some places. Much of this increase in thickness is due to the unexpected amount of deformation the ice has gone through, from repeated collisions between the chunks of sea ice forcing it to pile up and be pushed further under the water.

The increase in sea ice extent around Antarctica has been predicted reasonably well by climate models, according to the National Snow and Ice Data Center (NSIDC). However with these new findings, the sea ice around the continent may be growing more rapidly than we thought. Since Antarctic sea ice grows as a result of both losses from glacial ice on the continent and from the spread of the sea ice by strong winds from the southern polar vortex, the higher volumes may indicate higher loses from the continent, or other processes that need to be taken into account in climate modelling.

"The full 3-D topography of the underside of the ice provides a richness of new information about the structure of sea ice and the processes that created it," study co-author Dr. Guy Williams, from Institute of Antarctic and Marine Studies, said in a British Antarctic Survey statement. "This is key to advancing our models particularly in showing the differences between Arctic and Antarctic sea ice."