Why is there so much Antarctic sea ice?

Photograph from a ship traveling through Antarctic sea ice

Even during the Antarctic summer, heavy sea ice conditions are not uncommon. This photograph of sea ice was taken from the British icebreaker, HMS Protector, on its way to assist a Norwegian cruise ship that had become stuck in sea ice in January 2013.
–Credit: Royal Navy Media Archive (http://www.flickr.com/photos/rn_topten/8443836559/in/set-72157632684027667)

In late December 2013, the Russian research vessel, Akademik Shokalskiy, became trapped in thick sea ice off the coast of Antarctica. After several research vessels and icebreakers attempted rescue, the 52 passengers were evacuated. Soon after, one of the rescue ships also became stuck in the ice. However, conditions eased and both icebound ships safely churned out to open water.

Research in polar regions is inherently risky, and these events show how easily weather and ice conditions can disrupt research missions and travel during the already short Antarctic summer. But why was there so much sea ice around Antarctica to begin with, and why was it so thick? Antarctic sea ice is ruled by very different systems than Arctic sea ice. The reasons behind this increase are complex, and several recent studies show that scientists are still trying to understand them.

Ice and wind

Photograph of people on sea ice near pressure ridges

Sea ice floes bump into each other while floating on the ocean surface. The collisions form pressure ridges like the ones in this photograph.
–Credit: Eli Duke (http://www.flickr.com/photos/80547277@N00/4544002598/)

Part of the answer may be found in changes in atmospheric circulation linked to the Antarctic Oscillation, or Southern Annular Mode, which influences the large belt of air flows encircling the South Pole, called the circumpolar vortex. This oscillation varies on a decadal basis, alternating between negative and positive phases. Over the past few decades, it has shifted to more positive phases. “Positive phases are associated with a strengthening of the circumpolar vortex and intensification of the westerly winds,” said Jinlun Zhang, senior oceanographer at the University of Washington Polar Science Center. More intense winds have been whipping the Antarctic continent and battering the sea ice. “Ice floes converge in an area and cause an ice pile up, particularly along coastal areas,” Zhang said. More forceful collisions cause the ice to pile up along the floe edges, creating pressure ridges and producing thicker ice.

Map of former Mertz Glacier Tongue, off the coast of Antarctica

Two research vessels became icebound along this portion of the Antarctic coast in late 2013 and early 2014. This map shows the Mertz Glacier Tongue prior to February 2010, when it was broken off after a large iceberg collided into it. The tongue protruded 100 kilometers (62 miles) into the ocean, and prevented much of the sea ice from drifting west (toward the right in this image).
–Credit: United States Geological Survey

This particular winter, the icebound ships were also simply caught by bad weather and changes in regional conditions, according to an analysis by the National Oceanic and Atmospheric Administration. Sea ice in this area has increased after the recent loss of the Mertz Glacier Tongue. In 2010, a large iceberg collided with the ice tongue, breaking off a feature that had once prevented sea ice from drifting west along the coast. Ships now have a more difficult time breaking through the extra ice off this portion of Antarctica. In addition, thick sea ice is also snaring icebergs. In fact, a trapped iceberg temporarily blocked one of the rescue ship’s routes to open water.

Warming and wind

Antarctica’s windier climate may have another surprisingly distant source—in ocean currents half a world away. Researcher Xichen Li and his colleagues looked at the Atlantic Multidecadal Oscillation, a large-scale pattern in North Atlantic sea surface temperature that shifts between cool and warm phases. Similar to El Niño and La Niña, this oscillation affects temperature and rainfall worldwide, and now scientists think it is impacting Antarctic climate.

The North Atlantic is warming and staying warm, setting up far-reaching atmospheric patterns that affect the Antarctic Oscillation. In combination with the year-to-year influences from El Niño and La Niña, this pattern tends to intensify the westerly winds around Antarctica. It could also help explain regional differences around Antarctica: sea ice is increasing in some areas, while decreasing in others.

A disrupted heat flux

Additional culprits are the rising atmosphere and ocean temperatures around Antarctica. But how does warmer air and water create more sea ice? Overall warming alters the ocean heat flux, or the heat exchange between ocean, sea ice, and atmosphere, which typically regulates sea ice production.

Data image showing sea ice coverage around Antarctica, from 1979-2012

This data image shows the fractional ice coverage per decade, calculated for the period 1979–2012. The bold lines enclose areas where the change is statistically significant at the 5 percent level. The positive phase of the southern hemisphere polar vortex can affect sea ice volume and extent, but recent studies also suggest that the changes in ice cover and winds may also be caused by increased temperatures in the tropical and north Atlantic Ocean.
–Credit: J. King, 2014, Nature; data is from the National Snow and Ice Data Center, Boulder, Colorado; image is from the British Antarctic Survey

As deep ocean temperatures around Antarctic rise, they increase ice shelf melt, according to a study led by Richard Bintanja. This meltwater is creating a cool layer near the surface of the ocean that promotes sea ice production. In addition, the meltwater is fresh, or much less salty and dense than surrounding saline ocean layers. So fresher meltwater floats upward, mixing with the cold surface layer, lowering its density. As this fresh layer expands, it forms a stable puddle on top of the ocean that makes it easier to produce and retain sea ice.

This growing fresh puddle changes the ocean heat flux. Zhang also studied this change, and his models showed that warming would increase sea ice, up to a point. “If the current warming continues, the increase in ice may continue for some time,” Zhang said.  But, the increase will likely not continue indefinitely. “If the warming gets stronger, there will come a point when ice growth is smaller than the ocean heat flux available to melt ice,” he said. Under those conditions, sea ice extent and volume will begin to decrease.

Charting a complex environment

All of these explanations reveal how complex the Antarctic environment is, and how numerous factors affect its sea ice regime. Scientists continue to investigate the reasons behind Antarctica’s increasing sea ice, examining possibilities both close to home and further afield. As the studies show, most trends are small and mean different things than the Arctic sea ice trend, but scientists also agree this isn’t a signal of non-warming in Antarctica.

For now, most of the research missions delayed by the sea ice rescue have resumed, and will help expand what scientists know about this remote continent and the sea ice that surrounds it.

References

Bintanja, R., G. J. van Oldenborgh, S. S. Drijfhout, B. Wouters, and C. A. Katsman. 2013. Important role for ocean warming and increased ice-shelf melt in Antarctic sea-ice expansion. Nature Geoscience, doi:10.1038/NGEO1767.

National Oceanic and Atmospheric Administration. How unusual were the Antarctic sea ice conditions that trapped a research ship on Christmas Eve 2013? Accessed January 23, 2014.

King, J. Climate science: A resolution of the Antarctic paradox. Nature 505: 491-492, doi:10.1038/505491a.

Li, X., D. M. Holland, E. P. Gerber, and C. Yoo. 2014. Impacts of the north and tropical Atlantic Ocean on the Antarctic Peninsula and sea ice. Nature 505: 538-542, doi: 10.1038/nature12945.

Zhang, J. 2013. Modeling the impact of wind intensification on Antarctic sea ice volume. Journal of Climate 27: 202-214, doi:http://dx.doi.org/10.1175/JCLI-D-12-00139.1.

Zhang, J. 2007. Increasing Antarctic sea ice under warming atmospheric and oceanic conditions. Journal of Climate 20: 2,515-2,529. doi:http://dx.doi.org/10.1175/JCLI4136.1.

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